Ph.D. Student - Earth System Science (UC Irvine), B.Sc. - Atmospheric Sciences (Cornell University)
By: Zachary Labe , 7:45 PM GMT on May 08, 2008
"Thoughts of Flood Threat of May 8-13"
This is my specially formated new blog for flood threats in particular. Let me know what you think of it. More updates coming throughout the evening. Interesting setup for heavy rains over Pennsylvania, especially in the southern and eastern regions. Looking into the future a coastal storm will hit Friday, another for Sunday night, and another possible for Thursday. Each one will be accompanied heavy rain adding to potential totals of over 6inches in isolated locations by the time all of these storms exit the region. Today was a dreary day for much of Pennsylvania with around .1inches of rain falling statewide. I recieved .11inches today. This total will be getting increasingly higher over the next couple of days. Cloud ceilings rose in eastern Pennsylvania and allowed for a little sun to come out. But in western and northern Pennsylvania airfields at Bradford and Johnstown are reporting IFR ceiling conditions. More moist air is keeping cloud bases low. Becareful traveling tonight if going over mountaintops above elevations of 1500ft. Dense fog will be widespread. Conditions should remain IFR all throughout the day Friday with clearing conditions for Saturday with only isolated showers and thunderstorms. Models overall seem to be in pretty good agreement with this first storm and associated front. This blog is for just the first storm, but the HPC forecast precipitation map does include the second storms also. It appears as of 500pm that the rain is the Ohio and Tennesse Valley is getting its act together along with showers and embedded thunderstorms developing in extreme southern Pennsylvania and northern Maryland. The rain shield should be light to moderate at first. But by Friday heavy rain will occur in the south with embedded thunderstorms enhancing the rainfall. There will be a sharp-cut-off to the heavy rain though and that should setup in the northern Laural Highlands to Middle Susquehanna Valley to southern Poconos. Continue to check back this blog for more updates. Have a great evening!!!
(Courtesy of NOAA)
(Courtesy of Penn State Meteo.)
A stationary front draped along the Pennsylvania/Maryland border will be the culprit for the heavy rain. A weak sheild of precipitation moved through last night and this morning with the initial weak wave. Now a strengthening low pressure is developing in the Tennesse Valley and is going to ride up along the front. Strong southernly Gulf moisture winds will intensify drawing the moisture northward. PWATs are very high and go over 3deviations above normal. The airmass is relatively dry at this point, but as the low level jet noses northward moisture levels will be on the increase. Upper level CAPE values are relatively high, especially for the dreary day across southern Pennsylvania, but low level CAPE values are low. So expect thunderstorms to be low-topped and heavy rain producers. Thunderstorms, capable of severe limits will develop to our south in West Virginia and Virginia, but they will weaken as the move northward. The shield of precipitation will expand northward, but with a sharp cut off point. The low pressue will strengthen and move out in the Delmarva and then up the coast. Winds should be relatively light though. Overall the synoptic situation is excellent for heavy rain across southern Pennsylvania. The axis of heaviest rain should be in the southeast Piedmont to Atlantic Coastal Plain in Pennsylvania.
(Courtesy of NOAA)
"12hr Estimated Precipitation"
(Courtesy of NOAA)
"Forecast from Hydrometeorological Prediction Center"
POTENT SHRTWV OVR THE LWR OH VLY THIS AFTN WL CONT TO LIFT ENEWD
AND OFF THE MID ATLC CST FRI NGT. ASSOCD SFC LOW WL ALSO TRACK
NEWD ALNG A FNTL BNDRY WITH MDLS INDICATG MOD/STG CVRG AHD OF THE
LOW. A BROAD REGION OF UPR DIFL/DVRG UNDER A JET COUPLET IS ALRDY
IN PLC AND IS FCST TO INCRS TNGT...WHICH WL PROVIDE A BROADER
REGION OF DEEP LYRD LIFT. DEEP MSTR IS ALRDY FUELG SOME HVY RNFL
OVR THE TN/OH VLY REGION AND THIS MSTR WL ADVECT NEWD TNGT...WHILE
INCRSG. STRNGTHNG/BACKG LOLVL FLOW WL PULL ADDTNL MSTR NWD TNGT
WITH MDLS INDICATG PWS POOLG TO ARND 1.50 INCHES OVR THE MID ATLC
REGION. THE COMBINATION OF THE STG LOLVL FLOW...8H WNDS NR 50
KTS...AND DEEP MSTR WL SUPPORT UNSEASONBLY HI MSTR FLUX ANOMALIES
OF 4-5 STDS ABV NRML. GFS IS THE ONE SOLU THAT LOOKS TOO FAR
N...WHILE MUCH OF THE OTHER GUID IS FARTHER S AND MORE CONSISTENT.
GFS ERRORS SEEM TO BE LINKED TO FEEDBACK. GFSP IS MORE LIKE THE
12Z CAN/00Z ECMWF. LATEST SREF SOLUS ARE MORE INLN WITH THE
NON-GFS SOLUS. XPCT A SWATH OF HVY RNFL FM THE SRN APLCHNS THRU
THE MID ATLC REGION WITH A STRIPE OF VRY HVY AMTS ACRS NRN VA INTO
MD WHERE ISOLD 3-4 INCH TOTALS ARE PSBL.
"HPC Forecast 5-day total QPF map"
Models have been doing a pretty decent job when it comes to forecasting this storm. The GFS picked up on the storm, followed by the EURO. Then of course the NAM found it and used its over estimated precipitation bias to forecast the storm. The GFS and NAM print out a good 2+ inches of rain for a widespread area, but I believe that is due to convective feed back problems. It is picking up on the thunderstorm convection. I believe 1inch will be widespread, but there still will be some 2+ inch amounts especially in southern Pennsylvania/Maryland border counties. The SREF model continues to show CAPE values up to 400 in southern Pennsylvania. But anyone in the Middle Susquehanna on northward there is little to no CAPE for those regions. The NMM model is doing a pretty good job I believe with the storm, though I think the track is a little to far south, but it does pick up on areas of banding on potential training thunderstorms. The ARW model is doing also a similar to job to the NMM model, but it is a little farther north. Overall I believe the ARW model has the best grab on this storm. Below you can find the GFS and NAM total precipitation as of 12z Thursday for out to 60 hours. And below you can find the NMM simulated future radar for 9am Friday morning. In conclusion the models all have a very simular track with only a few little different variables.
"GFS model total precipitation"
"NAM model total precipitation"
"NMM model future simulated radar at height of storm"
"12hr Flash Flood Guidance"
(Courtesy of NOAA)
"My Flood Forecasts"
Ok, well with all of the synoptic factors coming together for a stalled front heavy rain event, flooding may be a slight threat. Now I believe an axis of heavy rain from Latrobe, PA to Somerset, PA to Gettysburg, PA to Harrisburg, PA to Reading, PA to Allentown, PA will recieve heavy rain. To the north of that line 25miles will be heavy rain. To the south of that line to the Pennsylvania/Maryland border will also be part of the axis of heavy rain. In that axis 1-3inches of rain is possible. In parts of York, Lancaster, Chester, Franklin, Adams counties up to 4inches of rain could fall. The big questions with this storm are how far north the shield will go and how much embeded thunderstorm activity can be expected. Currently in Virginia are building severe thunderstorms that are producing tornadoes in the western part of the state. The storms will move northeast, but gradually weaken as they more into more stable air. But none the less a stronger storm cannot be ruled out in the southern PA counties. For example in relatively stable air 2 tornadoes occured out in Ohio today with this system. Keep your weather radio on tonight. But overall I do expected a very limited threat. Now above were the flash flood guidance maps and amounts are pretty high that need to be reached in a 12hr period for flooding so for the first batch of rain only some urban and stream flooding can be expected. But under any areas of training thunderstorms, then flash flooding may occur. Flood watches are out for areas in southern Pennsylvania. Below are my rain map and flood map predictions. The only area I am concerned for bust potential is in northern Pennsylvania where less rain may fall than I predict. But as of now I will make no changes. More updates from my forecasts will be coming tomorrow. Stay tuned...
"My Forecast Total Rain Map"
"My Flood Risk Map"
"Conclusion on Flood Threat"
Overall for the first low pressure system little widespread flooding is expected. Just minor urban and creek flooding should be expected. But if some training thunderstorms can occur then a slight chance of flash flooding may occur. But come the second potential storm the flooding threat increases. And again a third potential storm behind that one. Creek levels will rise along with river levels. Moisture for soil anomalies remain 1-1.5deviations above normal, but with the lack of rainfall over the last few days combined with low dewpoints conditions were pretty dry. Beaware of low-lying areas and remember to turn around and not drown, especially at night. Also stay tune to latest statements from the NWS as flood warnings may have to be issued. Also keep a level on creek and stream levels over the next couple of days. I will try to post a map of state stream/river levels across Pennsylvania after the first storm. Flooding is the most deadly type of weather that hits the United States each year, and many of those fatalities can be prevented as long as people are aware of their surroundings and stay tuned to their local media. Have a wonderful day!!!
COMING THIS EVENING!!!
"Here north of Harrisburg 2008 statistics"
(Severe Weather Stats...)
Severe Thunderstorm Watches- 0
Severe Thunderstorm Warnings- 0
Tornado Watches- 0
Tornado Warnings- 0
Total Thunderstorms- 9
Flood Watches- 3
Flood Warnings- 3
Monthly Precipitation- .58inches
Yearly Precipitation- 14.24inches
Heat Advisories- 0
Excessive Heat Warnings- 0
90degree days- 0
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.
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