The Pennsylvania Observer

Transcription

1 The Pennsylvania Observer January 5, 2016 December Pennsylvania Weather Recap By: Arthur Person December is the first of the three coldest months of the year. Sometimes it can be naughty, and sometimes it can be nice. As for this 12th month of 2015, it will go down in history as one of the nicest Decembers on record thanks to an El Niño-influenced jet stream that persistently stayed to the north and west of Pennsylvania, keeping warmer than normal conditions present for most of the month. The month started off near normal as a storm system in the Plains buoyed warm moist air over chilly New England air producing widespread rain across all but the northwest corner of the state on December 1 st and 2 nd. As the storm retreated, clouds and showers persisted across the northwest on the 3 rd, and clearing skies prevailed for the 4 th -7 th with temperatures near normal. The 8 th and 9 th remained seasonal, but some clouds compromised sunshine. As the storm track and jet stream shifted northwest of Pennsylvania on the 10 th and winds shifted to the south, a dramatic warm-up began as temperatures reached record levels from the 12 th -14 th, despite significant cloud-cover. Some notable records set for the 13th include Scranton 66 F (previous 54 F), Philadelphia 71 F (previous 64 F) and Williamsport 62 F (previous 56 F). On December 14 th, Bradford broke its high temperature record by 10 F reaching 64 F. A storm crossed the state on the 14 th bringing light rainfall and cooler temperatures. However, temperatures remained above normal across the state under mainly cloudy skies through the 17 th when another cold front with moderate rainfall and chilly northwest winds finally brought temperatures down to near normal for the 19 th through the 21 st. By the 21 st, however, a storm track through the Great Lakes and a meandering frontal boundary led to rainy and warm conditions with several more record high temperatures on the 27 th. Altoona reached 71 F (previous 64 F) and Philadelphia reached 65 F (previous 64 F). As a cold front shifted eastward on the 27th, frigid high pressure to the north invaded the state setting the stage for freezing rain across northern portions of Pennsylvania on the 28 th and 29 th as moist air on easterly winds overspread the state. The chill did not last as temperatures rose above 40 F by late on the 29 th. Cloudiness and showers persisted across the state for the last two days of the month and temperatures remained above normal. December 2015 will be remembered for the remarkable warmth across the state. Several locations recorded above normal temperatures for the entire month (e.g. Middletown and Philadelphia) while the coolest locations only saw up to 5 days with below normal temperatures (e.g. Pittsburgh and State College). Widespread record-breaking high temperatures were recorded on 5 days of the month, and correspondingly, snowfall across the state was very low. Pittsburgh recorded its 3rd lowest snowfall for

2 December of only 0.3, and many locations tied their all-time records with only a trace of snowfall for the month (e.g. Harrisburg, Philadelphia, State College, Stevenson Dam, Williamsport). Here are the weather extremes across Pennsylvania (observations taken at 8AM EDT) during December 2015 from the NWS Cooperative, ASOS, and CoCoRaHS Networks of which our office receives routine observations. The extremes occurred in the 24-hour period prior to the date listed. Parameter Location Value Date (8 AM EDT) County Highest Temperature Wolfsburg 73 F December 28 th Bedford Lowest Temperature Chandlers Valley 1 Mi. SE 11 F December 11 th Warren Greatest Cumulative Liquid Precipitation West Chester 6.20 December 1 st -31 st Chester Least Cumulative Liquid Precipitation Greatest Cumulative Snowfall Tunkhannock 4.0 Mi. NW Springboro 3.0 Mi. WNW 2.29 December 1 st -31 st Wyoming 14.3 December 1 st -31 st Crawford Links to Pennsylvania Weather Stories during December 2015 Warm weather takes a toll on local ski resorts Allergies more of a problem than normal for early winter Unusual warmth affects crops across state

3 The Pennsylvania Observer FEATURED CLIMATE HIGHLIGHT By: Kyle Imhoff ENSO What is it and how do we measure it? El Niño and La Niña terminology that is regularly utilized by scientists, the media, and the general public to describe climatic events that play a major role in influencing atmospheric patterns across the globe. As frequently as these terms are used, however, an explanation of what these terms mean or how these events are measured are far less common. Over the past few months, strong El Niño conditions have emerged that have received a great deal of attention. This has piqued the interest of not only the science community, but non-scientists who are curious about the details. This month s climate highlight will focus on what ENSO means and how we measure this phenomenon.

4 Definition ENSO is defined as the El Niño Southern Oscillation. The name refers to two observed climatic phenomena that are directly related to each other. El Niño refers to anomalous sea-surface temperatures in the eastern equatorial Pacific Ocean. Temperatures that are anomalously warm in the equatorial Pacific are referred to as an El Niño event (what we are experiencing now in the latter half of 2015/early part of 2016). Conversely, temperatures that are relatively cool in this region are called La Niña events. The second half of the moniker is the Southern Oscillation. The Southern Oscillation refers to oscillations in atmospheric pressure differences between two tropical locations Darwin, Australia and the island of Tahiti. These pressure differences are an atmospheric response to the oceanic changes observed during La Niña or El Niño events. How scientists measure these two phenomena will be discussed next. Measurements Now that the definition of ENSO has been covered, it is important to understand how atmospheric scientists measure these phenomena. The Southern Oscillation is measured using the Southern Oscillation Index (SOI). It is a very straightforward measurement of the difference between mean sea level pressure (MSLP) at Tahiti and MSLP at Darwin. These pressure differences are then standardized 1 so that average or neutral conditions are centered at 0. Positive values of the SOI correspond to La Niña-like conditions whereas negative values indicate El Niño-like conditions. While there is one basic way in which the Southern Oscillation is observed, scientists measure El Niño and La Niña conditions over the Pacific Ocean in different ways. This highlight will focus on two different methods of quantification. The first is the Multivariate ENSO Index (MEI). As the name implies, multiple atmospheric and oceanic variables 2 are used to calculate index values, where positive values indicate El Niño-like conditions and negative values La Niña-like conditions. In the definition section above, it was stated that El Nino and La Nina were defined solely by sea-surface temperatures over the Pacific Ocean. This index, however, tries to capture the near real-time interactions between the atmosphere and the ocean in an attempt to more accurately assess the state of these climatic events. The second method of quantification, and the one which is most widely used and cited, is the Oceanic Nino Index (ONI) created by the Climate Prediction Center (CPC). This index uses a quantification method which strictly focuses on sea-surface temperatures over the eastern equatorial Pacific Ocean 3. The ONI averages sea-surface temperature anomalies (departures from the average) over a 3-month time period. The CPC restricts the use of the term El Niño event or La Niña event until certain criteria are met 4. These are just two indices which are an attempt at accurately assessing the state of ENSO at any given time. This highlight did not discuss the different regions of the equatorial Pacific Ocean that can be used to compute sea-surface temperature anomalies, which adds additional complexity to these

5 ENSO observational datasets. These differences lead to some disagreements within the scientific community of how, exactly, to define El Niño and La Niña events. But, generally these methods tend to overlap and support each other in determining the state of ENSO. 1 See the mathematical formulas for standardization: 2 Explanation of MEI calculation: 3 More details on how ONI is calculated and historical values: 4 CPC considers El Niño or La Niña conditions to occur when the monthly Niño3.4 OISST departures meet or exceed +/- 0.5ºC along with consistent atmospheric features. These anomalies must also be forecasted to persist for 3 consecutive months. Most recent ENSO outlook and more information:

6 The Pennsylvania Observer LONG RANGE OUTLOOK By: Kyle Imhoff The month of December was relatively warm across much of the continental United States east of the Rockies, particularly the Northeast. The Southwest was the only region to see slightly below normal readings for the month. The same areas that were warm were also relatively wet in December, with the Upper Midwest experiencing precipitation amounts well over 200% of normal. Outside of the typically-dry Southwest, there were no regions that experienced unusually dry conditions for the month, so only three anomalous regions will be used for the analog forecasts presented below. Using the three anomalous regions described above, climate records were examined for the 25 years in each region that historically are closest to what was seen in After identifying common years in the three anomaly categories, the years with the most categories in common and highest rank are used as analogs. Below is a table containing a list of years in which the anomalies in past Decembers matched what was seen in 2015.

7 Warm Cool Wet Northeast Southwest Upper Midwest The colors in the table above indicate years in common among all three anomalous regions yellow indicates two of the regions share that specific year and orange indicates all three in common. Composite maps for the months of December and January using the analog years in the table above are shown below.

8 The above images shows that the center of warmth will be over the Upper Great Plains and Midwest during the months of January and February. Pennsylvania temperatures are near normal during this time. Precipitation anomalies are below normal across much of the Ohio Valley and western Pennsylvania, while eastern Pennsylvania and the East Coast are near to above normal levels. Using the analogous years after 1948, below are forecast temperature anomalies for January and February for western, central and eastern Pennsylvania, respectively.

9 1/1 1/3 1/5 1/7 1/9 1/11 1/13 1/15 1/17 1/19 1/21 1/23 1/25 1/27 1/29 1/31 2/2 2/4 2/6 2/8 2/10 2/12 2/14 2/16 2/18 2/20 2/22 2/24 2/26 2/28 3/2 Mean Temperature Departure from Normal (deg F) Western Pennsylvania Temperature Forecast January - February Forecast 5-Day Mean Forecast Verification 5-Day Mean Verification Normal

10 1-Jan 3-Jan 5-Jan 7-Jan 9-Jan 11-Jan 13-Jan 15-Jan 17-Jan 19-Jan 21-Jan 23-Jan 25-Jan 27-Jan 29-Jan 31-Jan 2-Feb 4-Feb 6-Feb 8-Feb 10-Feb 12-Feb 14-Feb 16-Feb 18-Feb 20-Feb 22-Feb 24-Feb 26-Feb 28-Feb 1-Mar Mean Temperature Departure from Normal (deg F) Central Pennsylvania Temperature Forecast January - February Forecast 5-Day Mean Forecast Verification 5-Day Mean Verification Normal

11 1-Jan 3-Jan 5-Jan 7-Jan 9-Jan 11-Jan 13-Jan 15-Jan 17-Jan 19-Jan 21-Jan 23-Jan 25-Jan 27-Jan 29-Jan 31-Jan 2-Feb 4-Feb 6-Feb 8-Feb 10-Feb 12-Feb 14-Feb 16-Feb 18-Feb 20-Feb 22-Feb 24-Feb 26-Feb 28-Feb 1-Mar Mean Temperature Departure from Normal (deg F) Eastern Pennsylvania Temperature Forecast January - February Forecast 5-Day Mean Forecast Verification 5-Day Mean Verification Normal