Ph.D. Student - Earth System Science (UC Irvine), B.Sc. - Atmospheric Sciences (Cornell University)
By: Zachary Labe , 9:33 PM GMT on July 14, 2010
After a period of dry weather and near term isolated drought conditions, a recent change in the jet stream pattern has aided in a relocation of the heat ridge allowing for cold fronts to penetrate across the northern Middle Atlantic and washing out to the south in Virginia. 500mb analysis indicates a relocation of the Bermudian high slightly farther south and east in the Atlantic. But above normal temperatures will continue across the region courtesy of global abnormal thermal heights. Cold air is very absent across much of the globe, especially in the north Arctic where sea ice conditions are down at a record negative pace.
The upcoming pattern will continue favor above normal temperatures, but slightly enhanced precipitation in the rainfall department courtesy of an anomalous flow out of the Gulf of Mexico. Recent heavy rain during the past three days is courtesy of enhanced PWATs originating from the south with a near +2SD. SREF guidance quickly pinpointed to the higher 700mb moisture anomalies several days in advance of the heavy rain. Local groundwater conditions continued to heed below normal values especially in localized areas across northcentral Maryland and parts of the middle Susquehanna Valley in Pennsylvania where drought conditions on the Palmer Index entered D1. With soil moisture anomalies well below normal, the threat of flash flooding actually increases as the water washes quick away into urban areas and is not allowed to soak into the ground. FFG at the time was a general 4in/12hr across much of the state, but global model guidance indicated a heavy rain threat several days in advance. GFS meteograms for KMDT indicated nearly 1.5in of rain with local 4km WRF output near 6in for parts of southeastern Pennsylvania, likely courtesy of convective error. Monday, 14 July 2010, produced an interesting weather day across much of central Pennsylvania where near stationary convection developed on an axis of instability on a surface front ahead of the general cold front. PWATs near 2in with CAPE values near 2000j/kg produced a complex of thunderstorms on the leeward side of the Alleghanies with a general storm track and momentum to the east-northeast at 75degrees.
30knot wind shear 0-6km aloft allowed high echo tops from thunderstorms to bring down some higher wind gusts. Widespread wind damage was reported along a narrow corridor along the lower and middle Susquehanna River. General damage reports were courtesy of wet downbursts and straightline winds. Heavy rainfall rates were also reported ranging from 4-7in/hr for short periods of time courtesy of tapping into near tropical air aloft in the lower troposphere. Rainfall totals from the training thunderstorms were reported ranging from 1-4in. These complexes of thunderstorms later tracked into eastern Pennsylvania, but in a slow general weakening state by evening.
As the potent upper level trough lifts out of the region, the general shortwave will move offshore. Taking a near tropical depression outlook on local satellite imagery, the deformation axis of precipitation will move offshore by Wednesday evening with additional QPF generally .1in for eastern Pennsylvania. High pressure will return with an anti-cyclonic flow, but the airmass has non-polar origins therefore continuing the above normal temperature pattern. As the 1020mb high pressure moves offshore by Thursday evening, the clear skies featured on Thursday will be invaded by higher cirrus by 6z Friday. 2m temperatures look to be a general 90-95F for most locations in Pennsylvania, Maryland, and Delaware below 1000ft. Across the higher elevations such as the Laurels, Poconos, and Northwest highs will be a less salty 80-85F.
12z 7/14/10 GFS indicates an approaching cold front by Friday as a 1004mb low pressure moves through Ontario. A bubble of higher H85 thermals near 20C will center over the northern Middle Atlantic for Friday with very warm temperatures topping well over 90F for most locations outside the higher elevations. Southern urban locations such as Washington DC will feature highs in the upper 90s with widespread haze and poor air quality. 2m dewpoints are estimated to be regionwide in the 70s for a very oppressive day. Heat indices will approach 100-110F during the 11am-4pm heat of the day.
NAM/GFS QPF fields indicate the highest probability of rain occuring in northwestern portions of Pennsylvania. Increasing SBCAPE Friday afternoon near 2000j/kg with winds aloft near 40knots will allow for stronger convection to accompany the front in broken lines. EHI values approach 1-2 will also allow for the threat for a few rotating low levels in the strong thunderstorms. The highest threat for severe weather on Friday exists for northwestern Pennsylvania, but as convection seeps southwards Friday night, isolated wind damage may still occur as far south as northern Maryland. Not all areas will receive rain with southern areas progged at below .1in QPF. The latest 7/14/10 18z NAM remains especially dry for areas south of I-80.
Of interesting note, the GFS has recent gone under a major upgrade and this will take effect in the coming week. First expected to debut in June, recent delays in enhancing resolution has caused a slight later date.
TECHNICAL IMPLEMENTATION NOTICE 10-15
NATIONAL WEATHER SERVICE HEADQUARTERS WASHINGTON DC
853 AM EDT FRI APR 2 2010
EFFECTIVE JUNE 22 2010...BEGINNING WITH THE 1200 COORDINATED
UNIVERSAL TIME /UTC/ RUN...THE NATIONAL CENTERS FOR
ENVIRONMENTAL PREDICTION /NCEP/ WILL UPGRADE THE GLOBAL
FORECAST SYSTEM /GFS/. THE RESOLUTION OF THE GLOBAL FORECAST
MODEL WILL BE INCREASED FROM T382 /35 KM/ TO T574 /27 KM/.
THE HIGH RESOLUTION PORTION OF THE FORECAST WILL BE EXTENDED
FROM 180 HRS TO 192 HRS. WITH THIS EXTENSION 3 HOURLY OUTPUT
WILL BE MADE AVAILABLE OUT TO 192 HOURS... Link.
A seconday cold front is progged by latest GFS to move across the northern Middle Atlantic and wash out over northern Maryland. This will allow for another day of convection, some of which could be severe. Strong areas of lift in the right front entrance of the low level jet will allow for higher winds aloft to be reached by strong thunderstorm echo tops. Highs will once again reach 90F for many areas outside the higher elevations, but will be 4-6F cooler than Friday. Humidity levels remain high with PWATs near 2.0in. Models diverge on solutions for the beginning of next week, with the ECMWF bringing the cold front boundary northward as a warm front as early as Sunday with accompanying showers, but the GFS delays this northward progression. In any case unsettled conditions will continue through the middle of next week with a boundary located over the northern Middle Atlantic. This will allow for daily diurnal convection and accompanying threats of flash flooding. Meanwhile the fast zonal jet streamal flow will promote a rapid progression of a strong shortwave early next week across the northern tier. For now this ring of fire looks to be north of the northern Middle Atlantic, but details remain uncertain. At this time, it appears the threat of a widespread wind damage outbreak from a derecho is possible across the northern tier United States. SPC already highlighting the threat in their long term outlook...
ZCZC SPCSWOD48 ALL
ACUS48 KWNS 140850
SPC AC 140850
DAY 4-8 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0350 AM CDT WED JUL 14 2010
VALID 171200Z - 221200Z
...SEVERE WEATHER EPISODE FROM THE NRN PLAINS ACROSS THE GREAT LAKES
AND INTO THE NERN STATES POSSIBLE DAYS 4-6...
MEDIUM RANGE GUIDANCE IS COMING INTO BETTER AGREEMENT MOVING A SHORT
WAVE TROUGH RAPIDLY EWD ACROSS THE NRN TIER OF A STATES FROM ND
ACROSS THE GREAT LAKES AND TOWARD NEW ENGLAND DURING DAY 4 /JUL 17/
THROUGH DAY 6 /JUL 19/. LATEST SEVERAL RUNS OF THE GFS AND ECMWF
SHOW QUITE SIMILAR TIMING AND AMPLITUDE OF THE SYSTEM...AND ARE
CONSISTENT WITH THE 12Z ECMWF ENSEMBLE MEAN SOLUTIONS. STRONG LOW
AND MID LEVEL JETS ARE FORECAST TO TRANSLATE EWD IN ASSOCIATION WITH
THE FAST MOVING FRONTAL SYSTEM...WITH GUIDANCE INDICATING AMPLE LOW
LEVEL MOISTURE/INSTABILITY AND STRONG VERTICAL SHEAR ACCOMPANYING
THE SYSTEM. SEVERE STORMS ARE EXPECTED TO DEVELOP DURING DAY 4 /JUL
17/ OVER THE ERN DAKOTAS AND SPREAD EWD ACROSS THE UPPER MS
VALLEY...WITH THE SYSTEM CONTINUING EWD ACROSS THE GREAT LAKES AREA
ON DAY 5 /JUL 18/. BY DAY 6 /JUL 19/ THE CONVECTIVE SYSTEM IS
EXPECTED TO CONTINUE EWD ACROSS PARTS OF THE NERN STATES. OVERALL
JET PATTERN SUGGESTS POTENTIAL FOR A MORE WIDESPREAD DAMAGING WIND
EVENT DURING THIS PERIOD ALTHOUGH IT IS TOO EARLY TO PROVIDE DETAILS
OF THE CONVECTIVE EVOLUTION.
For the time being the excessive dry and hot weather will remain absent from the northern Middle Atlantic allowing for a more active weather pattern to resume. Heavy rain and near term flash flooding will be primary threats in the coming seven days. Recent Mt. Holly rainfall analysis indicates excessive rainfall over much of the eastern half of the northern Middle Atlantic.
Heavy rains have also impacted the Philadelphia metropolitan region...
As of 5 PM... Philadelphia International Airport reported nearly 3 inches of rain today... 2.88 inches to be exact... most of that occurring in a 2 hour period this afternoon.
A quick recap for the forecast for the coming few days across most areas in the northern Middle Atlantic...
Thursday- General sunshine with high pressure dominating, but temperatures remain mild in the lower 90s.
Friday- A shortwave and accompanying cold front approach from the northwest with an increase in humidity and heat. Thunderstorms are also possible over the region with the highest threat during the late afternoon over northwestern Pennsylvania. A few storms may be severe. Highs in the mid 90s.
Saturday- A seconday cold front approaches with more showers and thunderstorms with mostly cloudy skies and highs in the upper 80s to low 90s.
Sunday- Chance of spotty showers/thunderstorms with mostly cloudy skies and highs in the 80s.
Regional updating radar...
"Here north of Harrisburg 2010 statistics"
(Severe Weather Stats...)
Severe Thunderstorm Watches- 6
Severe Thunderstorm Warnings- 7
Tornado Watches- 1
Tornado Warnings- 0
Total Thunderstorms- 13
Flood Watches- 3
Flood Warnings- 1
Monthly Precipitation- 4.09inches
Yearly Precipitation- 21.19inches
Heat Advisories- 2
Excessive Heat Warnings- 0
90degree days- 22
Highest Temperature- 101F (x2)
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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