Ph.D. Student - Earth System Science (UC Irvine), B.Sc. - Atmospheric Sciences (Cornell University)
By: Zachary Labe , 12:35 PM GMT on July 22, 2008
"Thoughts of Flood Threat of May 8-13"
Many areas across eastern Pennsylvania have been relatively dry lately for a few thunderstorms occuring though Sunday evening. Some of those storms produced rainfall totals near .5inch along with some wind damage in the region as the cold front passed through. Also on Monday a few rain showers and thunderstorms occured in the northern parts of Pennsylvania producing locally heavy rainfall. Rainfall has been especially plentiful over western and northern Pennsylvania where thunderstorms have been occuring almost everyday in the past 7 days during the heat wave as instibility and orographic lift produced pulse thunderstorms that produced very heavy rain. Now last night a MCV pulled of Ohio and created numerous thunderstorms again in western Pennsylvania. As these storms tracked east they organized into a large cluster producing areas of heavy rain. They got as far east as Harrisburg, Pennsylvania in which a few hundreths of an inch of rain was recorded. The ground is pretty moist from Harrisburg-Selinsgrove-Williamsport line on westward. Now a positively tilted trough will move through the Great Lakes. As it moves through in will deepen down to the Tennessee Valley and become negatively tilted as it becomes somewhat stalled over the eastern portions of the northeast and middle Atlantic states. For a more detailed setup see the section below. As training thunderstorm develop behind a MCS that moves through Tuesday night, heavy rain will break out. Widespread amounts of 1-2inches looks likely from central Pennsylvania on eastward. But locallized amounts could approach 12hr flash flood guidance (see below). The NWS is contemplating posting Flash Flood Watches and it looks likely that they will do so for eastern areas. Also severe weather may be a threat as we are in the right entrance quadrant of the jet stream containing winds to near 40knots in the mid levels. Helicity values are slightly high so even rotating storms cannot be ruled out. CAPE values get to near 2000 j/kg on Tuesday also. Wind damage looks to be the primary threat. Also for Wednesday more strong thunderstorms could form, but will be somewhat limited thanks to cloud debris from a Wednesday morning MCS over southeastern Pennsylvania. So quite a busy weather couple of days and it will be quite interesting to see how it all plays out. Stay tuned for more updates. I thought the best idea to handle this situation would be my flood threat formatted blog. Have a great day!!!
(Courtesy of NOAA)
(Courtesy of Penn State Meteo.)
A cold front is drifting southeastward followed by a steep upper level trough. The trough will be deepening and dropping to as low as the Tennessee Valley, which is quite impressive for late July standards. This puts central and eastern Pennsylvania in the right front quadrant which helps to enhance thunderstorm and severe thunderstorm formation. Upper level winds are an astonishing 100knots, but more reasonable in the mid levels from only 35-4knots which is still adequate enough for severe weather formation. A MCS is rolling currently across western Pennsylvania and this complex will be the focus of much of the activity today. There is a slight risk of severe weather over the entire region today thanks to the latest SPC outlook. The MCS last night produced a plethura of severe weather across the Mississippi and Ohio Valley and now has weakened to just heavy rain and lightning. The low level jet continues to keep the MCS together with high moisture and cold cloud temperatures. Though as it moves into a more CIN environment this morning in eastern Pennsylvania it should weaken before strengthening again this afternoon and tap into the winds aloft producing some severe weather. CAPE values increase to near 2500 j/kg in extreme southern Pennsylvania. As the front slowly moves across the state a shortwave will rotate along it bringing nondiurnal instibility aiding in the formation of another strong MCS over northern Maryland and southern Pennsylvania. This is when helicity values increase making there a potential for an isolated rotating storm. This complex will produce heavy rain and move out of the region by Wednesday morning. As moisture increases on Wednesday and PWATs rise several deviatons to potentially over 2inches several rounds of thunderstorms will form from central Pennsylvania on eastward. Tropical moisture left over from Cristobal will move into the region along with Gulf of Mexico moisture riding up the nearly stationary cold front. Training thunderstorms could lead to flash flooding on Wednesday with some locallized areas possibly receiving an amount near 5inches when everything is all said and done. Severe weather is also a slight threat on Wednesday, but there should be widespread debris clouds to keep a hold on severe weather development. Damaging winds would be the primary threat. The SPC does put eastern portions in a slight risk. Overall the event is very convective type rains in nature. Some areas may see little rain, while others see a lot of rain making quite a difficult forecast.
(Courtesy of NOAA)
"12hr Estimated Precipitation"
(Courtesy of NOAA)
"Forecast from Hydrometeorological Prediction Center"
...CONCERNING THE SLOWLY DIGGING GREAT LAKES UPPER TROF...
VERY DYNAMIC SITUATION EXPECTED TO SET UP FROM THE MID ATLC COAST
NWD INTO THE NE AS PIVOTING S/WV THRU THE OH VALLEY UNDER CLOSED
LOW OVER THE GT LAKES WILL ALLOW ENTIRE UPR TROF TO TAKE ON A STG
NEG TILT LATER THIS AFTN AS IT PUSHED EWD TOWARD THE MID ATLC
REGION. THIS WILL RAPIDLY INCREASE THE UPPER DIFFLUENCE OVER
ASSOCD SLOW MOVING COLD FRONT AS WAVELENGTH BETWEEN THE S/WV AND
UPR RIDGE HOLD FIRM OFF THE EAST COAST SHORTENS. LOW LEVEL SLY
JET WILL RESPOND IN KIND..WITH INCREASING SLY LOW LEVEL JET ALONG
AND AHEAD OF THE FRONT AND POOLING OF 1.75 TO 2 INCH PLUS PWS.
THE COMBINATION OF STG UPR FORCING AND VERY FAVORABLE LOW LEVEL
MSTR..INSTABILITY AND FRONTAL CONVERGENCE SHOULD RESULT IN IN A
RATHER WELL DEFINED SQLN LATER THIS AFTN AND OVERNIGHT THAT WILL
MOVE SLOWLY EWD. NAM PERHAPS LOOKS A LITTLE SLOW WITH THE EWD
PROGRESSION OF THE UPR TROF..SO LEANED A BIT TOWARD A COMPROMISE
IN SPEED BETWEEN THE GFS AND NAM. OVERALL..SHOULD SEE SOME VERY
HEAVY RAINFALL RATES GIVEN THE ABOVE WITH SOME GENL 1 TO 2 INCH
AMOUNTS PER HR WITH STORM TOTAL AMOUNTS OF 2 TO 4 INCHES WITH
ISOLD HEAVIER AMOUNTS PSBL FROM NRN VA NWD INTO ERN NY AND WRN
PORTIONS OF NEW ENGLAND.
"HPC Forecast 5-day total QPF map"
As the models have been they are suffering from major convective feedbrack, the HPC is also mentions this. Rainfall is probably grossely overestimated in many areas, but still locallized areas may see those amounts. The GFS model is what my forecast appears similar to indicating the Tuesday morning MCS, the Tuesday night MCS, and the training thunderstorms on Wednesday. One issue I see is it might be a little far to west with the cutoff heavy rain line in central Pennsylvania. The NAM seems to be having even more convective feedback issues and sort of shows another scenerio in which there is no second MCS. Instead training thunderstorms develop over western Pennsylvania and move eastward across the state by Wednesday afternoon. Now one thing I am worriful is I have found that models greatly overestimate QPF sometimes with storms. But still they have been indicating this heavy rain threat for a long period of time. And already there forecasts and placements of MCS I think have been pretty good. Also with slow moving thunderstorms, PWATs near 2inches, and previous rainfall somewhere there will likely be flooding even if the forecasts do not come out quite according to plan. It seems the bullseye in Pennsylvania is in the northeastern part of the state in the Poconos.
"GFS model total precipitation"
"NAM model total precipitation"
"12hr Flash Flood Guidance"
(Courtesy of NOAA)
"My Flood Forecasts"
Well were are already beginning to see training thunderstorms forming over northwestern Pennsylvania. That will be similar to what will happen Wednesday except it will be much more widespread. I think the highest potential for flooding will be in the poconos. The flood map below indicates potential for flooding threats. The map does not mean flooding will occur; it shows which areas have the highest potential for some sort of flooding. Also another higher risk area is in southwestern Pennsylvania and the southern Laural Highlands. That is because some areas have already seen 2inches of rain in the last 12 hours and that orographic lift from the tall mountains will enhance rain totals. Also below is my rainfall accumulation map in which all of Pennsylvania should see over .5inches. But with convective rains there will probably somewhere that does not see that much rain. Across southern Pennsylvania and up through eastern Pennsylvania is where widespread 1-2inches should occur with isolated amounts of 4inches may occur due to training thunderstorms. I will post verification maps for both maps after the storms is over. Stay tuned for more updates throughout the day.
"My Forecast Total Rain Map"
"My Flood Risk Map"
"Conclusion on Flood Threat"
Well overall it looks mainly like a flash flood threat and not river flooding and not widespread flooding. Make sure you stay tuned to the radar screens and advisories issued by the NWS. This could be a serious threat for low-lying areas. Already this morning Erie, Pennsylvania is getting pounded with heavy rain and thunderstorms. Also there is some severe weather beginning to occur in New York State and the Delmarva region. Stay tuned also for the severe weather threat to see how it unfolds later today. Have a great day!
Very interesting storm moved across the region from June 22-24. It all started with a series of complexes of thunderstorms that developed in the Ohio Valley and tracked eastward spelling severe weather in western Pennsylvania. This all occured several days leading up to the major storm. Even a 71mph wind gust was recorded in Pittsburgh. The MCSs would track eastward across central Pennsylvania and finally dissapate towards the Susquehanna River. I only received .03inches of rain from the MCS. Then as a deep upper level trough moved into the region, the low level jet was positioned in which the central to eastern Pennsylvania zone was in the right front quadrant. First early Wednesday morning severe thunderstorms stationed along a warm front in Virginia began to lift northward. As the storms spread across the Mason-Dixon line a very intense line of thunderstorm built up. Severe warnings went up from Fulton County all the way to Chester County as a line of thunderstorms developed and lifted northward. Intense and vivid lightning also occured with the storms, but most of the lightning was Cloud to Cloud lightning limiting the amount of loud thunder other than a steady low rumble. Winds also accompained the line with wind damage reported in York, Cumberland, and Juniata Counties. Rainfall was also heavy with the line and most places received from .4-.8inches of rain with the initial line. By later in the morning these storms moved up through the Poconos and gradually weakened. Then as the jet stream built in more low level moisture with rising PWATs near 2inches a light to moderate rain band formed in central Pennsylvania and moved from south to northnortheast at a very slow rate. Rainfall amounts in the band were generally around .3inches initially. As the band tracked eastward the trough became negatively tilted now pumping up moisture. A series of weather events took place that great resembled winter storms to cause heavy rain. Thunderstorms began to train, especially from the Middle Susquehanna Valley up through the Poconos. Rainfall totals were in excess of 5inches. Flash flooding occured. Meanwhile along the front in western Pennsylvania severe thunderstorms formed and slowly tracked eastward. Flash flooding occured with this line near Monroeville in Alleghany County and also wind damage was reported. Shear levels aloft were quite high near 40knots the closer to the trough. Helicity values were very high making tornadoes a threat. Kinematics were extremely higher than normal which made up for the weak thermodynamics and instibility. CAPE values were pretty meager in most areas, excluding Philadelphia which they were near 2000 j/kg CAPE. As the second band began to weaken in eastern Pennsylvania. The third line of severe storms took over and marched eastward, but then began to weaken. Severe pulse thunderstorms then began to form in extreme eastern Pennsylvania and resembled supercells causing damage in Lehigh and Bucks Counties with hail and high winds. Very heavy rain fell in these thunderstorms. By later in the evening as the final weather events came together a intense line of training thunderstorms formed from Maryland up through southeastern Pennsylvania in the Piedmont region. This line began to expand and dumped the last of the rains. Meanwhile back in northwestern Pennsylvania backlash rain and thunderstorms unexpectedly formed causing flash flooding in Warren and Mckean counties were mudslides were reported along with water rescues. The rains finally wrapped up by early Thursday morning with amounts in the Piedmont near 3inches, Lower Susquehanna Valley near 2inches, Middle Susquehanna Valley near 3inches, Central Pennsylvania near 1inch, northwestern and western Pennsylvania near 4inches isolated 6inches, and Poconos up to 6inches. Overall the impacts of the storm were pretty significant but flooding reports were pretty minimal. Most of the heaviest of rains occured in areas that had been recently dry. The storm very much resembled a winter storm with even backlash snows around the low pressure. Pretty amazing. Thursday afternoon also some instibility northwest thunderstorms formed causing some severe hail reports across the central Mountains near State College. And early this Friday morning lows dropped from 45degrees in Bradford to mid 50s elsewhere. What a storm!
Flood Verification Map...
My flood map fared ok during the event. The worst of the flooding was in two locations. One in the poconos which I highlighted in my outlook and the second area in the northwest mountains which I put a low risk of flooding. Still overall I am pretty pleased.
Rain Fall Verification Map...
My rainfall total map did very well throughout the event. Convective rains are hard to forecast, because thunderstorms can hit one area and completely miss another area. That is why I usually do not issue rain maps during thunderstorm outbreaks. But anyway overall the only change that I would have made is in the northwest mountains where I could have upped estimated rain fall totals. Overall I am very pleased as the worst of the rains where in the areas which I highlighted in my outlook.
"Here north of Harrisburg 2008 statistics"
(Severe Weather Stats...)
Severe Thunderstorm Watches- 5
Severe Thunderstorm Warnings- 3
Tornado Watches- 1
Tornado Warnings- 0
Total Thunderstorms- 25
Flood Watches- 4
Flood Warnings- 5
Monthly Precipitation- 3.47inches
Yearly Precipitation- 27.85inches
Heat Advisories- 4
Excessive Heat Warnings- 0
90degree days- 10
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