Forecasts for Wilmington, NC 1-4 November 2013. Name: Joe Sample This paper documents weather forecasts issued for Wilmington, NC on Friday November 1 for the period 1-4 November. It examines forecasts made by the following sources:

• the NAM computer model • the GFS computer model • the National Weather Service • AccuWeather (as reported in Wilmington’s daily newspaper, The StarNews) • WECT TV (as reported on their web site)

The setting The forecasts were rather consistent regarding the major weather-producing feature: an approaching cold-front during Friday, November 1 (Fig. 1) and its passage Saturday morning, November 2 (Fig. 2). Each forecast called for rain beginning on Friday night and ending on Saturday morning. Each predicted unseasonably warm, humid conditions with strong and gusty southwest winds on Friday as the front approached. These are the expected conditions in a maritime tropical air mass ahead of the cold front. Every one of the forecasts anticipated showers, rain, or

thundershowers in association with the approaching front as the warm moist air would be lifted. The computer models and National Weather Service (NWS) issued probabilities ranging from an 85% to 100% chance for some precipitation. Figures 3 and 4 show the 850-mb maps on Friday and Saturday. The strong SW flow on Friday shows how moisture was delivered from the Gulf of Mexico, and how on Saturday, the winds shift to W & NW cutting this supply. Figures 5 and 6 show the 700-mb map at 1:00am and 8:00am Saturday. The colored areas indicate high upward vertical velocity in the high moisture areas. The combination of moisture and upward motion led forecasters to expect ample precipitation from the storm. Details on precipitable water are found in the NWS Forecast Discussion below.

Fig.  2.  NWS  forecast  map  valid:  Sat.  8pm Fig.  1.  NWS  current  map:    Fri.  Nov.  1

Fig.  3.  NAM  model:    850mb  Friday Fig.  4.  NAM  model:    850mb  Saturday

Fig.  5.  NAM  model:    700mb  Sat.  1am Fig.  6.  NAM  model:    700mb  Sat.  8am

The forecasts The five forecasts (issued on Friday) for temperature and precipitation during the 4-day period were all very close to one another, and for the most part, close to the observed values (Table 1). Every source accurately predicted Friday’s high temperature 8 hours in advance. Longer-lead temperature forecasts for Sat-Mon also were predicted very well. Exceptions include the NAM model prediction for Saturday’s low temperature and the overestimated Saturday high temperatures predicted by both NAM and GFS models (meteorologists seemed to do better than the models as expected). The precipitation forecasts are all close to the actual amount received. The models predicted precipitation amounts as well as the National Weather Service. Forecast  source   1  Nov  

Tmax  2  Nov  Tmin  

Precipitation  1-­‐2  Nov  

2  Nov  Tmax  

3  Nov  Tmin  

3  Nov  Tmax  

4  Nov  Tmin  

4  Nov  Tmax  

NAM  model   80 61 0.52-1.17” 76 47 67 GFS  model   82 65 0.52-1.08 ” 77 47 67 National  Weather  Service   82 65 0.36-0.82” 73 47 64 44 61 Wilminton  Star  News[paper]  (AccuWeather)   80 66 75 48 65 41 60 WECT  TV  (Wilmington)   79 65 74 47 65 41 63 Observed   81 64 1.03” 71 46 66 41 63    The timing of the frontal passage was excellent. The meteograms below show the NWS forecast for temperature, dewpoint, wind speed and direction, clouds, and precipitation potential. Collectively, these graphs show the expectation for a front passage sometime around 6:00am EDT. The radar images to the right demonstrate that the forecasted precipitation did, in fact, occur when the models and NWS thought it most likely (midnight to 6:00am EDT). Further evidence that all forecast sources did well is found in the two table for the computer model (NAM and GFS) variables as compared to observed changes at Wilmington. Variables highlighted in red (in the model output table) and in the red rectangle (in the observed meteogram) indicated evidence of the frontal passage.  

 

KILM NAM MOS GUIDANCE 11/01/2013 0000 UTC DT /NOV 1 /NOV 2 /NOV 3 / HR 06 09 12 15 18 21 00 03 06 09 12 15 18 21 00 03 06 09 12 18 00 X/N 80 61 76 47 67 TMP 71 71 71 77 79 76 75 73 68 65 63 70 74 73 63 58 53 50 48 65 55 DPT 67 66 66 66 66 67 67 67 64 61 58 57 50 47 48 46 44 42 41 40 43 CLD BK BK OV BK BK OV OV OV OV OV OV BK SC SC BK CL CL CL CL CL CL WDR 19 20 22 22 21 21 20 21 22 25 28 31 29 29 29 30 32 33 34 36 03 WSP 07 11 12 14 15 13 09 10 14 10 05 09 09 09 05 04 05 06 06 09 04 P06 41 19 29 76 75 10 2 3 4 4 5 P12 41 85 16 4 6 Q06 1 0 0 4 3 0 0 0 0 0 0 Q12 1 4 0 0 0 T06 11/ 4 18/12 7/12 32/11 25/ 8 2/ 6 2/10 1/ 9 1/ 1 0/ 1 T12 18/12 32/14 25/ 8 2/10 1/ 1 SNW 0 0 CIG 8 8 6 6 6 7 6 4 4 3 6 8 8 8 8 8 8 8 8 8 8 VIS 7 7 7 7 7 7 7 5 5 5 5 7 7 7 7 7 7 7 7 7 7 OBV N N N N N N N BR BR BR BR N N N N N N N N N N KILM GFS MOS GUIDANCE 11/01/2013 0600 UTC DT /NOV 1 /NOV 2 /NOV 3 / HR 12 15 18 21 00 03 06 09 12 15 18 21 00 03 06 09 12 15 18 00 06 X/N 82 65 77 47 67 TMP 70 79 81 78 75 73 71 69 66 70 74 72 63 57 54 51 49 59 65 54 45 DPT 66 67 66 67 66 67 68 65 61 57 51 47 47 46 44 42 40 39 35 37 35 CLD OV BK BK OV OV OV OV OV OV OV BK BK SC FW CL CL CL CL CL CL CL WDR 20 21 21 21 20 20 21 23 28 32 30 28 29 31 34 35 35 36 34 01 03 WSP 10 16 16 11 09 12 12 10 08 08 09 08 05 05 05 07 07 14 11 05 08 P06 33 43 62 69 44 18 9 7 0 0 1 P12 71 92 53 14 0 Q06 0 1 2 2 1 0 0 0 0 0 0 Q12 1 4 1 0 0 T06 5/ 5 14/10 27/ 7 36/20 4/ 2 1/ 5 2/ 7 1/ 2 0/ 4 0/ 3 T12 33/10 40/20 3/10 1/ 5 0/ 4 POZ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 POS 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 TYP R R R R R R R R R R R R R R R R R R R R R SNW 0 0 CIG 8 8 8 8 7 4 4 4 5 8 8 8 8 8 8 8 8 8 8 8 8 VIS 7 7 7 7 7 7 7 5 6 7 7 7 7 7 7 7 7 7 7 7 7 OBV N N N N N N N BR BR N N N N N N N N N N N N  

Wilmington  Observed  Meteogram  

 

 NWS:    http://www.weather.gov/ilm/  

 Wilmington  Newspaper  

 http://www.accuweather.com/en/us/wilmington-­‐nc/28401/daily-­‐weather-­‐forecast/329819  

 Local  Television,  WECT  

 http://www.wect.com/weather  

 Sources National Weather Service forecasts, observations, and discussion: http://www.weather.gov/ilm/> and <http://weather.uwyo.edu/surface/meteorogram/> Figures 3-6: http://weather.unisys.com/forehome.php Computer model output: <http://www.nws.noaa.gov/mdl/synop/products/bullform.all.php> Computer model explanations: <http://www.nws.noaa.gov/mdl/synop/mavcard.php> and, http://web.gccaz.edu/~lnewman/GPH212_Forecasting/GPH212_MOS_Help.html

Wilmington area Forecast Discussion National Weather Wervice Wilmington NC 706 am EDT Fri Nov 1 2013 Synopsis... A strong cold front approaching from the west will move across the area saturday morning. it will become windy ahead of this front with a high risk for showers and possibly a thunderstorm. Significant rainfall is possible. high pressure will follow late Saturday through Sunday bringing with it colder and drier conditions. High pressure will persist through the middle of next week. Near term /through tonight/... As of 700 am friday...A surface cold front extending south of the low is now entering the southern and central appalachians. Tropical moisture is present with precipitable water values expected to exceed 2.0 inches by this evening just ahead of the front. To put that in perspective...2.0 inches is the 99th percentile for Nov. precipitable water for Charleston, SC and could be a record November pw value at Wilmington. The front will make steady eastward progress...and the band of showers and embedded t-storms should slide into the central carolinas this afternoon. Forecast pops along and west of interstate 95 are in the 60-90 percent range by late afternoon...with the most substantial coverage of precipitation expected along the coast after dark where my pops are 80-90 percent. storm total rainfall could exceed 1 inch...with heavier amounts possible if/where convective training occurs. compared to my thoughts based on the 00z models...the 06z models plus the latest sref slows the whole system down by a few hours. Temperatures today should be very warm by november standards...courtesy of tropical air advected northward ahead of the front. 850 mb temps expected to rise to +15c are in the 95-98th percentile for november. temperatures should leap with the first touch of sunlight this morning...and highs should reach 80-82 degrees for areas east of interstate 95 and a few miles inland from the beaches. the front will move across the area late tonight with cold advection not reaching the coast until just a few hours before daybreak. there is some uncertainty with tonight`s low temperatures since weaker winds behind the front could allow for boundary layer decoupling just before sunrise...but it appears lows should range from the mid 60s on the coast to the upper 50s inland. Short term /saturday through sunday night/... Cold front will be making its way off shore sat morning. dry and cooler air will begin to feed into area behind the front but deep nw flow with greatest caa will come later on sat as mid to upper trough finally swings through. may see some clouds with its passage but think most clouds and any chc of a sprinkle will remain to the north where better shortwave energy will be. After a warm start to the day on sat...temps will reach into the 70s as skies clear from west to east through the day. 850 temps plummet through sat into sunday morning dropping down to 2 to 3c by sun. dewpoint temps will drop out as well with reading in the lower 30s by mon morning. expect temps to drop a good 10 degrees sat night from previous min temps and then an additional drop sun night in better radiational cooling with readings right around 40.