Early Guidance for Tropical Weather Systems

Transcription

1 Early Guidance for Tropical Weather Systems Understanding What s Out There Jennifer McNatt National Weather Service Material from: Dan Brown, Mike Brennan and John Cangialosi, National Hurricane Center

2 Which of the following is not true for Invests a. Allows NHC to collect additional data and run model guidance b. Implies that the system is likely to develop c. Model guidance is not always run for invests d. The initial location can be moved tens of miles from cycle to cycle

3 What is an Invest A weather system for which a tropical cyclone forecast center (NHC, CPHC, or JTWC) is interested in collecting specialized data sets and/or running model guidance. (In other words, something we want to investigate further.)

4 Caveats for Invests Opening an invest allows NHC to monitor disturbances more carefully: Collection of microwave data Ability to run model guidance No standard for opening an invest Guidance is typically run when a cloud system center is apparent More meteorological uncertainty associated with invests! Extreme caution should be used when looking at model plots for invests

5 Tropical Weather Outlook Tropical Cyclone Models Track and Intensity

6 Tropical Cyclone Weather Track Outlook Overview Track forecasting is a relatively simple problem Cork in a stream analogy Important atmospheric features that control track are relatively large and easy to measure 7

7 Tropical Cyclone Weather Models Outlook Statistical and Dynamical Statistical Tells you what normally occurs based on the behavior of previous storms in similar situations (i.e., storm location, time of year, current motion, intensity, environment) Dynamical Attempt to predict what will happen in this specific situation 8

8 Tropical Cyclone Weather Models Outlook Spaghetti Plots What does this set of lines represent? Do they accurately convey the uncertainty in the track forecast? Are they all created equal? What s missing? 9

9 Tropical Cyclone Weather Models Outlook Spaghetti Plots 11

10 Tropical Cyclone Weather Models Outlook Extrapolated Motion Useful if models aren t handling initial motion well in the very short term No utility beyond 6 or 12 hours at most 12

11 Tropical Cyclone Weather Models Outlook Climatology and Persistence Used as a baseline to compare other forecasts with Not used as a forecast tool 13

12 Tropical Cyclone Weather Models Outlook Global Models Best forecast models for TC track Developed for general weather forecasting Handle large-scale pattern and steering flow well Can t see details of TC inner core Sometimes struggle with storm structure and intensity, which can affect track forecasts 14

13 Tropical Cyclone Weather Models Outlook Regional Hurricane Models Developed specifically for TCs Higher resolution means they can potentially do better job of handling interactions between TC and environment Limited coverage means features far away from TC may not be handled as well, which can degrade longerrange forecasts 15

14 Tropical Cyclone Models Consensus Models Typically the best track guidance, especially if the member models all show a similar forecast scenario Doesn t work well when members forecast very different track scenarios 16

15 Tropical Cyclone Model Plots Models Spaghetti Plots Caution! People looking at publically available model track plots, aren t seeing the whole picture Some of the best guidance (ECMWF, FSSE) isn t publicly available for proprietary reasons No sense of continuity from cycle to cycle for the various models, trends, etc. Users/viewers don t have the forecaster s perspective and knowledge to know model strengths and weaknesses, trends, etc. 17

16 Consensus Models Consensus Example Examples Tropical Storm Cristobal (2014) Model errors are often random (e.g., small variations on a common theme) Consensus frequently cancels out these random errors, resulting in a better forecast 18

17 Consensus Models Consensus Example Examples Tropical Storm Cristobal (2014) Model errors are often random (e.g., small variations on a common theme) Consensus frequently cancels out these random errors, resulting in a better forecast 19

18 Consensus Models Consensus Example Examples Hurricane Joaquin (2015) HWFI GFDI GFSI TVCA ERGI EMXI Consensus approach doesn t always work, especially when model scenarios are completely different Sometimes the forecaster might want to exclude certain models and form a selective consensus, if the discrepancies among the models can be resolved 20 Resolving these discrepancies is very difficult

19 Track Model Verification Atlantic 2017 (Preliminary) Yearly Track Model Performance Trends OFCL very skillful and was near best-performing consensus models (HCCA, TCVA, FSSE) EMXI was best individual model at all lead times, but trailed OFCL and consensus EGRI and UEMI were next best models GFSI, HWFI, AEMI, and CMCI were fair performers NVGI, HMNI, and CTCI lagged 29 April National Hurricane Conference 22

20 Track Model Trends Yearly Track Model Performance Trends Best 48-h Track Model by Storm 2017 Considerable variability from storm to storm, with no clear best model at 48-h across the board EGRI: Gert, Harvey EXMI: Irma, Ophelia GFSI: Jose, Maria HWFI: Nate 29 April National Hurricane Conference 23

21 Track Model Trends Best 48-h Track Model Yearly Track Model Performance Trends Due to model changes and other factors, the best performing model often varies from season to season ECMWF (EMXI) was best model in 2017, just edging UKMET and HWRF A global model has been the best at 48 h every year since 2006 except April National Hurricane Conference 24

22 Track Model Trends Best 5-day Track Model Yearly Track Model Performance Trends Due to model changes and other factors, the best performing model often varies from season to season EMXI was best model at 5 days in 2017 and has been since 2015 years running HWFI was tied with EGRI for second place in 2017 A global model has been the best at 5 days every year since 2001 except one 25

23 Tropical Cyclone Models TC Track Models The NAM The NAM TC track errors from the NAM are about 50% higher than the GFS The NAM should not be used for TC forecasting 29 April National Hurricane Conference 26

24 TC Track Forecasting Continuity Forecast Challenges Large track forecast errors often result from the following scenarios 1. Low predictability in the large-scale steering pattern 2. Misrepresentation of TC structure in models, resulting in improper steering flow 3. Weak steering currents, resulting in track being driven by mesoscale or convective scale factors 27

25 Tropical Weather Outlook Two-Day Graphic Current location of disturbances (discussed in the Tropical Weather Outlook) Formation chance during the next 48 hrs Categorical (Low, Medium, and High) Probabilities

26 Tropical Weather Outlook Five-Day Graphic Formation potential during the next 5 days Initial location of disturbance (X) indicated Shading represents potential formation area Single disturbance-based graphics available to help when areas overlap

27 Potential Formation Area Not a 5-day Track Forecast Tropical Outlook 8am July 28 70% 2 Day 30% 5 Day 70%

28 Special Tropical Weather Outlook Issued anytime there are significant changes with respect to disturbances in the TWO. Can be updated for either the 2- or 5-day probabilities Most commonly updated when formation probabilities are too low Often used to report findings of a recon invest mission

29 Verification of TWO Probabilities 48 hour Low bias Forecasts very reliable. For example, when NHC has issued a 30% chance of formation of a disturbance within 48 hours, about 32% of the time they have become a TC within that time period. High bias

30 Verification of TWO Probabilities 5-day probabilities Forecasts generally wellcalibrated. Low bias Low bias for probabilities from % High bias

31 Potential Tropical Cyclone Advisories How Did We Make Due Without This Capability Allows timely issuance of watches and warnings before a tropical cyclone has formed

32 Summary of 2017 Pre-TC Watches/Warnings Storm Additional Lead Time* (hours) Bret 24 Cindy 21 Franklin 6 Harvey 6 Ten False Alarm Lidia (EPAC) 27 Maria 6 Philippe 21 Average 15.9 hours *Based on Operational Assessments

33 Watches and Warnings Before Tropical Cyclone Formation Issued only for systems threatening land within the watch/warning time frame. Initial advisory issuance is not directly tied to tropical cyclone formation chance. Initial issuance criteria include: Likely impacts Need for tropical cyclone watches or warnings Desire to avoid switching warning types (tropical vs. non-tropical)

34 Reminders of Messaging Considerations for Potential Tropical Cyclones Advisory packages will be discontinued when watches and warnings are no longer necessary. POTENTIAL TROPICAL CYCLONE ONE PUBLIC ADVISORY NUMBER 1 NWS NATIONAL HURRICANE CENTER MIAMI FL AL PM CDT WED JUN TROPICAL DISTURBANCE OVER THE EAST-CENTRAL GULF OF MEXICO EXPECTED TO BECOME A TROPICAL STORM......TROPICAL STORM WARNING ISSUED FOR PORTIONS OF THE WEST COAST OF FLORIDA... When the threat is not imminent this could result in occasional gaps in product issuance. SUMMARY OF 400 PM CDT UTC...INFORMATION LOCATION N 86.5W ABOUT 310 MI KM SW OF TAMPA ABOUT 320 MI KM SSW OF APALACHICOLA FLORIDA MAXIMUM SUSTAINED WINDS...35 MPH...55 KM/H PRESENT MOVEMENT...N OR 360 DEGREES AT 3 MPH...6 KM/H MINIMUM CENTRAL PRESSURE MB INCHES WATCHES AND WARNINGS CHANGES WITH THIS ADVISORY... A Tropical Storm Warning has been issued for the west coast of Florida from Boca Grande to Ochlockonee River. SUMMARY OF WATCHES AND WARNINGS IN EFFECT... A Tropical Storm Warning is in effect for... * The west coast of Florida from Boca Grande to Ochlocknee River A Tropical Storm Warning means that tropical storm conditions are expected somewhere within the warning

35 Potential Tropical Cyclone Messaging Considerations Earlier NHC advisories for systems that pose a long-range threat to the United States or other land areas. Forecasts likely to have greater uncertainty. Intensity forecasts are likely to be conservative. False alarms could: Reduce long-term effectiveness of watches and warnings Affect reputation & trust of NHC tropical cyclone forecasts

36 Pre-Harvey Timeline (Texas) NHC began mentioning potential hazards for Texas in 1 PM CDT Tuesday Tropical Weather Outlook, a little more than 24 hour before redevelopment occurred Storm surge TS or hurricane-force winds Heavy rainfall Mentioned likelihood of a TS or hurricane watch at 1 AM CDT Wednesday Hurricane and Storm Surge Watch issued at 10 AM CDT Wednesday when Harvey regenerated as a Tropical Depression If confidence in development and potential impacts to land are high, should NHC have the option to issue Potential Tropical Cyclone advisories before the watch phase? Would the potential for additional false alarms outweigh the need for watch, warning, and forecast information before formation?

37 Potential Tropical Cyclone Invest in New Terminology? Feedback suggests that Potential Tropical Cyclone wording is not well understood by the public Media indicated that it was difficult to communicate Suggested wording has included: Potential Tropical Storm Potential Hurricane Potential Tropical Threat Tropical Disturbance Others? Does the name Potential Tropical Cyclone cause public confusion? Is there a better naming convention?

38 Key Takeaways Invests say nothing about a systems development potential. Users should refer to the Tropical Weather Outlook! Genesis probabilities are reliable Low does not mean no! Hatched areas on the 5-day Graphical TWO represent the potential formation area not a true track forecast. NWS/NHC now able to provide appropriate lead time for watches and warnings for potential tropical cyclones!!

39 TC Track Models Summary Concluding Remarks Track Models Global models are the most skillful for TC track prediction Consensus aids are more skillful than most individual models, and often beat the official track forecast NHC forecasters have philosophical constraints on the official forecast that leads to a certain amount of response lag May contribute to forecast biases and slightly poorer performance than the consensus While it is possible to beat the models from time to time, model performance has improved significantly over the years, and they are very difficult to beat consistently 47

40 TC Track Models Summary Concluding Remarks Track Models Large track forecast errors often occur due to Uncertainty in large-scale atmospheric flow Uncertainty in TC intensity and structure Track guidance for invests should be treated with extreme caution 48

41 NHC Forecast Continuity Philosophy Forecast Continuity Previous official forecast exerts a strong constraint on the current forecast Credibility can be damaged by making big changes from one forecast to the next, and then having to go back (flipflop, windshield-wiper) Changes to the previous forecast are normally made in small increments We strive for continuity within a given forecast (e.g., gradual changes in direction or speed from 12 to 24 to 36 h, etc.) As a result, NHC official forecasts are often slower to reflect big changes than the model guidance 49

42 Rapid Changes Forecast Continuity Messaging Large Forecast Shifts Large shifts in the NHC track forecast are sometimes necessary, typically due to large shifts in the guidance This can be difficult to message, since the shift may occur over 2 or 3 forecast cycles Look for key messages in the TCD, and can be discussed by your local office or NHC TS Debby (2012) Cone Graphics Advisories 4 through 9 50

43 Rapid Changes Forecast Continuity Messaging Large Forecast Shifts THE TRACK FORECAST IS EVEN MORE COMPLEX. THE GFS INSISTS ON A TRACK TOWARD THE NORTHEAST AS DEBBY BECOMES EMBEDDED WITHIN A LARGE MID-LATITUDE TROUGH. HOWEVER...THE ECMWF AND THE HWRF BUILD A RIDGE TO THE NORTH OF DEBBY AND FORECAST A WESTWARD TRACK. GIVEN THE WESTWARD TURN INHERITED FROM THE PREVIOUS FORECAST...AS WELL AS THE HISTORICAL STRONG RECORD OF THE ECMWF...THE NEW OFFICIAL FORECAST MOVES DEBBY INITIALLY A LITTLE BIT TO THE NORTHEAST TO REFLECT CURRENT TRENDS BUT THEN TURNS THE CYCLONE BACK TOWARD THE WEST OR WEST-NORTHWEST IN 24 TO 36 HOURS. A MAJORITY OF THE GFS ENSEMBLE MEMBERS NOW ARE CONSISTENT WITH THE DETERMINISTIC RUN...WHICH WAS NOT THE CASE YESTERDAY...MAKING A STRONGER CASE FOR THE EASTWARD SOLUTION. WE MUST BE READY TO MAKE A CHANGE OF THE FORECAST TRACK AT ANY TIME. TS Debby (2012) Advisory 5 Discussion TS Debby (2012) Advisory 5 Cone Graphic 51

44 Rapid Changes Forecast Continuity Messaging Large Forecast Shifts THERE HAS BEEN A SIGNIFICANT CHANGE IN THE FORECAST TRACK WITH THIS ADVISORY. THE OFFICIAL FORECAST NO LONGER BRINGS DEBBY WESTWARD ALONG THE NORTHERN GULF OF MEXICO AND INSTEAD KEEPS THE CYCLONE MEANDERING OVER THE NORTHEASTERN GULF FOR THE NEXT 3 TO 4 DAYS. THIS FORECAST IS A COMPROMISE BETWEEN THE CONSISTENT EASTWARD SOLUTION PROVIDED BY THE GFS FOR THE PAST SEVERAL DAYS AND THE NEW TWIST OF THE ECMWF. THE ECMWF MODEL...WHICH HAS BEEN FORECASTING DEBBY TO MOVE WESTWARD ALONG THE GULF OF MEXICO... NOW HAS THE CYCLONE MEANDERING FOR THE NEXT 3 DAYS OVER THE NORTHEASTERN GULF. SINCE THESE TWO RELIABLE MODELS ARE IN MARGINALLY BETTER AGREEMENT...I AM A LITTLE MORE CONFIDENT...BUT NOT COMPLETELY...THAT DEBBY IS NOT GOING TO TURN WESTWARD OVER THE GULF. HOWEVER...NEW OFFICIAL TRACK REMAINS A LOW- CONFIDENCE FORECAST. TS Debby (2012) Advisory 6 Discussion TS Debby (2012) Advisory 6 Cone Graphic 52

45 Rapid Changes Forecast Continuity Messaging Large Forecast Shifts BEST GUESS AT INITIAL MOTION IS QUASI- STATIONARY. DEBBY REMAINS IN A COL REGION OF THE MID-TROPOSPHERIC STEERING FLOW BETWEEN TWO ANTICYCLONES...AND IS LIKELY TO REMAIN SO FOR THE NEXT COUPLE OF DAYS. THEREFORE LITTLE MOTION IS ANTICIPATED DURING AT LEAST THE FIRST HALF OF THE FORECAST PERIOD. IN THE LONGER-TERM...THE TRACK GUIDANCE CONTINUES TO BE ALL OVER THE PLACE...WITH SOME MODELS TAKING DEBBY WEST AND NORTH OF ITS CURRENT POSITION AND OTHERS MOVING EAST OR NORTHEAST AND ULTIMATELY INTO THE ATLANTIC. THE LATTER SCENARIO ASSUMES THAT DEBBY WILL EVENTUALLY BE INFLUENCED BY A MID-TROPOSPHERIC TROUGH OVER THE NORTHEASTERN UNITED STATES. THIS SCENARIO SEEMS MORE LIKELY SINCE IT IS SUPPORTED BY BOTH THE GFS AND THE ECMWF MODELS. REGARDLESS OF WHICH SCENARIO PLAYS OUT...THE CYCLONE DOES NOT SEEM TO BE GOING ANYWHERE ANYTIME SOON. TS Debby (2012) Advisory 8 Discussion TS Debby (2012) Advisory 8 Cone Graphic 53

46 Rapid Changes Forecast Continuity Messaging Large Forecast Shifts GLOBAL MODELS DEPICT AN AMPLIFYING TROUGH OFFSHORE THE EASTERN SEABOARD CAUSING DEBBY TO MOVE GENERALLY EASTWARD THROUGH THE FORECAST PERIOD...WITH THE CENTER EMERGING OVER THE SOUTHWEST ATLANTIC BY DAY 5. THE ECMWF/GFS SOLUTIONS BEGIN TO LIFT OUT THE WESTERN ATLANTIC TROUGH IN ABOUT 72 HOURS...WHICH COULD LEAVE DEBBY IN WEAK STEERING CURRENT AGAIN AFTER THAT TIME. SINCE YESTERDAY... THE GUIDANCE HAS COME INTO BETTER AGREEMENT...WITH ONLY THE UKMET NOW SHOWING A WESTWARD MOTION. IN ADDITION...THE LATEST GFS ENSEMBLE MEMBERS FAVOR AN EASTWARD MOTION BY ABOUT A 3 TO 1 RATIO. THE NEW FORECAST TRACK REPRESENTS A SUBSTANTIAL CHANGE FROM THE PREVIOUS ONE BUT IS STILL SLOWER THAN THE MULTI-MODEL CONSENSUS IN THE SHORT TERM. TS Debby (2012) Advisory 9 Discussion TS Debby (2012) Advisory 9 Cone Graphic 54

47 Are the NHC track forecasts improving?

48 Atlantic Track Error Trends preliminary

49 3 day average track errors 1990

50 3 day average track errors

51 Track Errors over the past 5 years In general, track errors increase about n mi per day

52 Average track errors

53 Average track errors

54 Average track errors

55 Average track errors

56 Average track errors

57 Average track errors Outside RMW, significant storm surge

58 Average track errors Direct hit, highest surge and strongest winds

59 Average track errors

60 Averages are nice to know, but each case is different

61 2017 Storms 5-year average

62 48-h Model Track Errors by Storm Considerable storm-to-storm variability

63 Track Errors by Intensity As the initial intensity of the storm increases, NHC track errors on average get smaller.

64 Represents probable track of tropical cyclone center Formed by connecting circles centered on each forecast point (at 12, 24, 36 h, etc.) Size of the circles determined so that, for example, the actual storm position at 48 h will be within the 48-h circle 67% of the time NHC Forecast Cone

65 2018 Atlantic Cone Forecast period (h) Circle radii (n mi) Change from % smaller % smaller % smaller % smaller % smaller % smaller % smaller

66 NHC Track Cone Questions Does the cone tell you about impacts? No Does the cone know if the system is big or small? No Does the cone know if the forecast is confident or highly uncertain? Nope

67 Will the NHC track forecasts continue to improve? Skill has been levelling off for the past few years.

68 Tropical Tropical Cyclone Intensity Overview Much more complex forecast problem than track Involves interactions between thunderstorms in the core, the environment, and atmosphere-ocean interactions Important factors Track Wind, temperature, and moisture patterns in the core and the near environment Internal processes, such as eyewall replacement cycles, that are poorly understood 76

69 Tropical Tropical Cyclone Intensity Overview Statistical models tell us normal behavior in a given situation Extremely difficult to forecast unusual or extreme changes in intensity Consequently, confidence is usually not high enough to show rapid changes in intensity in the official forecast Dynamical intensity models have typically lagged the statistical models, but HWRF and HMON both beat the statistical models in 2017 Intensity consensus aids typically shows the most skill Official forecast tries to maintain continuity with previous forecast, if possible 77

70 Intensity Model Verification Atlantic 2017 (Preliminary) Concluding Remarks Track Models OFCL skillful at all times, but trailed consensus models at most time periods FSSE best model from 24 to 72 h HWFI was a strong performer, best individual model HMNI not as good as HWFI, but beat statistical aids DSHP and LGEM were fair performers, but not as good as HWFI, HMNI, and consensus models CTCI showed increased skill with time, strong performer days 3-5 GFSI had some skill, but not competitive; EMXI not skillful 29 April National Hurricane Conference 78

71 Tropical Tropical Cyclone Intensity Challenges Rapid Intensification MH H Intensity guidance from first forecast when Harvey regenerated in the Gulf of Mexico (12Z 23 August 2017) Actual intensity increased 30 kt in 24 h and 65 kt in 48 h No guidance from this cycle showed Harvey reaching hurricane intensity before landfall Landfall intensity 115 kt (cat 4) 29 April National Hurricane Conference 79

72 TC Intensity Models Summary Concluding Remarks Track Models Forecasting rapid intensification remains a big challenge, but we are making some progress Regional dynamical hurricane models are now competitive with statistical approaches Proper specification of the initial vortex remains a big roadblock to improving dynamical model forecasts How to best use the data in the models? What do you use for storms with little in situ data? 29 April National Hurricane Conference 83

73 Are the NHC intensity forecasts improving?

74 Atlantic Intensity Error Trends , 80-89, Prelim Only small improvements between , but errors have decreased more sharply this decade.

75 Intensity Errors over the past 5 years Intensity errors increase for 3 days, then level off

76 So What WILL you see in NWS briefings before guidance is available?

77

78 Instead of Spaghetti Plots

79 Setting the Stage

80

81

82

83 We still struggle with words!

84 Verification Web Page

85 Questions? Thanks again to Dan Brown, Mike Brennan and John Cangialosi at the National Hurricane Center!!!!