000 
fxus61 kphi 210129 
afdphi 


Area forecast discussion 
National Weather Service Mount Holly New Jersey 
929 PM EDT Mon Aug 20 2018 


Synopsis... 
a front that has been stalled south of our region the last few days 
will lift northward as a warm front on Tuesday, followed by a cold 
front on Wednesday. High pressure builds in from the north and west 
to close out the work week. The high then moves offshore this 
weekend. 


&& 


Near term /until 6 am Tuesday morning/... 
for the evening update, I adjusted temps across the Lehigh 
Valley and Poconos, edging them down a degree or two. 


With the atmosphere quickly stabilizing this evening, I took out 
the small threat of convective showers and thunderstorms for 
the rest of the evening across our Delmarva zones. 


Previous discussion... 
stalled frontal boundary remains just off to our south this 
afternoon near Virginia with a series of disturbances moving 
through the weak upper level flow over the area. Plenty of 
moisture remains in the low levels near and north of the front 
with NE winds and this continues to result in considerable cloud 
cover helping keep temps on the cooler side. The northern edge 
of instability is across the Delmarva and as we go through the 
late afternoon into early this evening, there could be some 
scattered showers and storms in this area mainly from around 
Dover southward as indicated by some of the high res models. 
Elsewhere, skies remain overall mainly cloudy into this evening 
but conditions remain dry. 


After sunset, as we head through the mid to late evening and beyond 
any diurnal storms over the Delmarva diminish with more in the way 
of clear breaks possibly developing for a time as we lose the 
diurnal cu. However by the overnight low stratus should set in once 
again and there could even be a few light showers along with some 
patchy drizzle in spots due to weak lift through the relatively 
shallow moist layer. Also, as low pressure advances north and east 
toward the Great Lakes this will help slowly lift the front back to 
the north as a warm front...reaching near the Chesapeake Bay by 
morning. Lows overnight will generally range from the mid to 
upper 60s except low 60s across the southern Poconos. 


&& 


Short term /6 am Tuesday morning through 6 PM Tuesday/... 
Tuesday shaping up to turn active once again as low pressure 
continues to move north and east into Ontario. This will help slowly 
push the aformentioned warm front North/East into the area but its 
progress will be impeded by a surface ridge over New England 
extending into the mid Atlantic. The combination of the warm 
front along with diurnal instability and upper level forcing 
driven by the approaching upper trough will result in scattered 
showers and storms developing by late morning into the 
afternoon. These will tend to develop from SW to NE across the 
area as the front advances becoming more widespread with time as 
the afternoon GOES on. Instability may be limited, at least 
initially, due to the slow movement of the front so don't have 
big concerns for widespread severe threat. Bigger concern looks 
to again be Hydro issues as pwats rise to around 2.25 inches 
meaning any storms will be capable of producing very heavy 
rainfall with more flooding possible. However period of greatest 
concern looks to be tomorrow evening as the stronger forcing 
aloft along with a prefrontal trough at the surface moves 
through bringing more widespread coverage of storms. High temps 
will generally range from the upper 60s to mid 70s across the 
southern Poconos and northwest NJ, where the warm front may not arrive 
until tomorrow night, to the low 80s from around philly 
southward where it will also feel quite muggy again by 
afternoon. 


&& 


Long term /Tuesday night through Monday/... 
summary...active weather continues through at least early Wednesday 
as showers and thunderstorms will be possible in the vicinity of a 
cold front. Then by Wednesday night, a long awaited pattern change 
means that our region should have tranquil weather into early next 
week. 


Details: 


Tuesday night and Wednesday...cold front is expected to 
propagate through the region from late Tuesday night through 
Wednesday afternoon. In the mid and upper levels, a short wave 
trough will be digging southeast towards our region. Highest 
chances for showers and thunderstorms look to be ahead of the 
cold front, mostly Tuesday night into Wednesday morning. As for 
The Hazards through this period: 


Heavy rain: precipitable water values will once again be well 
above normal and we will continue to have a deep warm cloud 
layer. Thus, the potential exists for heavy downpours with any 
showers or thunderstorms through this period. Wet antecedent 
conditions (nearly saturated soils) continue across large 
portions of the region, so it may not take much to cause 
flooding in many areas. However, storm motions will be near or 
above 20 mph, so that should limit a widespread flooding threat. 
The main system itself is also rather progressive. For this 
reason, will hold off on a Flash Flood Watch for now, but will 
be carefully watching as this period comes within the 
operational periods of high resolution models. 


Severe weather: due to the timing of this event, primarily 
overnight into the early morning hours, instability will be 
limited. However, 0 to 6km bulk shear values could be 30 knots or 
more along and west of the fall line. Thus, the severe threat is 
low, but I can't completely rule it out at this point. 


Thursday through Saturday...a large high builds in over the 
region through this period, resulting in a multiple day dry 
period. Thanks to cold air advection behind the front, 
temperatures should be near normal, with highs in the 70s and 
80s. What is going to make even more of a difference is that dew 
points will be in the 50s across the region, making it feel 
much more comfortable. 


Sunday and Monday...the high shifts off shore. As a result, low 
level flow over our region shifts to southwesterly, and could 
Herald a return to warm and muggy conditions. A surface trough 
in this period could provide enough focus for precipitation 
development, but with zonal flow aloft, synoptic scale lift 
looks weak. Thus, widespread precipitation is unlikely. 


&& 


Aviation /01z Tuesday through Saturday/... 
the following discussion is for kphl, kpne, kttn, kabe, krdg, 
kilg, kmiv, kacy and surrounding areas. 


Tonight...cigs expected to deteriorate to MVFR with time...most 
likely within a couple hours of either side of midnight for 
most sites though exact timing of this is uncertain. Also, it's 
possible brief IFR could occur toward 12z with the best chances 
for this at krdg where we've indicated it as a tempo group. 
Winds generally east/NE 5-10 knots or less. Low confidence. 


Tuesday...MVFR cigs should continue to predominate through the 
morning. There may be some improvement to VFR in the afternoon 
however by late day showers and storms will be developing and 
any of these could bring additional restrictions. Winds 
generally southeast increasing to around 10 knots by afternoon with 
some higher gusts. 




Outlook... 


Tuesday night and Wednesday...periods of MVFR or lower 
conditions possible in showers and thunderstorms. Expect a wind 
shift to northwesterly Wednesday with a cold front. Gusts to 
near 20 knots are possible on Wednesday. Low confidence on the 
timing of lower conditions or thunderstorms, but moderate 
confidence on other aspects of the forecast. 


Thursday through Saturday...VFR conditions expected with light 
northwesterly winds becoming light and variable Friday into 
Saturday. High confidence. 


&& 


Marine... 
winds/seas expected to be below Small Craft Advisory levels through Tuesday. 
There will be continuing low stratus tonight into Tuesday with 
some areas of light showers and drizzle possible. Heavier 
showers and storms will approach by late day Tuesday with southeast 
winds increasing to around 15 knots with gusts to around 20 
knots. 


Outlook... 


Tuesday night and Wednesday...a period of Small Craft Advisory conditions, for both 
elevated seas and wind gusts around 25 kt, is likely starting early 
Wednesday morning continuing through Wednesday afternoon on the 
Atlantic coastal waters. On the Delaware Bay, Small Craft Advisory conditions are 
unlikely. 


Thursday through Saturday...winds and seas are expected to stay 
below Small Craft Advisory criteria. 


Rip currents... 


Low risk for rip currents are forecast for Tuesday. Onshore 
winds on Tuesday are forecast to be about 5 mph less than 
Monday. This will be coupled with a 2 to 3 foot swell rather 
than the 3 to 4 foot we saw on Monday. When combining both winds 
and swell, the rip current risk will be low. 


&& 


Phi watches/warnings/advisories... 
PA...none. 
New Jersey...none. 
Delaware...none. 
Maryland...none. 
Marine...none. 


&& 


$$ 
Synopsis...Johnson 
near term...fitzsimmons/kruzdlo 



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