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Area Forecast Discussion (AFD)

Issued by Philadelphia/Mt Holly (PHI)

FXUS61 KPHI 211455

Area Forecast Discussion
National Weather Service Mount Holly NJ
955 AM EST Thu Feb 21 2019

Low pressure over the Mid-Atlantic will lift toward the Gulf of
Maine this morning before finally departing late in the day. 
High pressure builds in from the north and west to close out the
work week, then moves offshore on Saturday. Low pressure lifts 
through the region over the weekend, followed by high pressure 
for the start of next workweek.


For this update...afternoon high temperatures were adjusted 
down 1 to 2 degrees across the southern 2/3rds of the forecast 
area. I also adjusted the sky cover grids. Lower clouds are 
exiting the region at a nice clip and are being replaced by some
higher cirrus. So filtered sun expected today with above normal
temperatures and a bit of breeze this afternoon.

Previous discussion...
Precipitation has ended at this hour as the primary low has 
moved north into Ontario with a secondary low taking shape off 
the New Jersey coast. However low levels remain saturated so 
patchy fog and drizzle may continue to be an issue into early 
this morning. And where temperatures remain below freezing 
across parts of eastern PA into northern NJ there will be the 
potential for some freezing drizzle. The good news is that by 
sunrise the low off the New Jersey coast will have moved 
northeast toward Cape Cod with high pressure beginning to nose 
eastward toward the Ohio Valley. This will help set up a 
pressure gradient with a westerly flow of drier air developing 
so any fog/drizzle will be diminishing around this time. Low 
stratus will stick around a bit longer though before mixing out 
by this afternoon with partly to mostly sunny skies developing 
along with breezy west winds. The airmass will remain quite mild
though so expect highs ranging from the 40s over the southern 
Poconos with most of the remainder of the area seeing highs in 
the low to mid 50s.


High pressure at the surface will build into the Great Lakes 
region tonight with ridging extending eastward toward the mid 
Atlantic. This will occur as an upper level ridge remains in 
place over the SE CONUS while an upper level trough remains in 
place in the west. A weak front will remain situated to our 
south and expect there will be some moisture moving NE along 
this feature. However any precip should stay suppressed to our 
south by the ridge though there will be some increasing mid and 
high level cloudiness. This will help keep temperatures from 
getting too cold so expect lows mostly ranging from the mid 20s 
to the mid 30s...coldest over the southern Poconos into the 
Lehigh Valley and mildest over southern NJ and the Delmarva.


Big picture:

The long term period will begin with a fairly similar synoptic
setup to what has been observed over the last couple weeks with
a high amplitude ridge over the Gulf of Alaska, a longwave 
trough over the western US, and subtropical ridging in place 
over the eastern US. A substantial piece of the longwave trough 
will eject NEwrd Saturday passing through the Great Lakes & Quebec
Sunday into Monday. A warm storm system and gusty postfrontal 
winds will impact the area in association with this system. As 
this shortwave passes the mean Atlantic ridge will flatten and 
relocate eastward. This ushers in at least a temporary pattern 
change as the mean trough propagates eastward towards the 
vicinity of Hudson Bay by the middle of next week. Although the 
specifics of what this temporary pattern change will bring us 
weather-wise are nebulous, this pattern tends to favor somewhat
cooler temperatures over the area with the potential for colder 
temperatures when the vortex sags southward. Precipitation-
chances are also largely dependent upon moisture from southern 
stream systems. 

Friday... High pressure over the upper midwest will propagate
eastward on Friday residing just to the north of the forecast
area by day's end. This will result in a quiet day over the area
with little sensible weather and seasonable temperatures. It
should be noted there is a largely stationary W-E oriented
boundary centered south of the area on Friday which serves as a
focus for precipitation. Guidance generally keeps any 
precipitation south of even Delmarva with this setup so did not 
add mentionable PoPs for Friday, but did add some cloud cover 
over far southern Delmarva. 

Saturday-Sunday morning... The high should crest the area 
Saturday afternoon with near zonal flow aloft backing more 
southwesterly in advance of the NEwrd propagating shortwave 
over the Central Plains. Once the high moves offshore Saturday 
PM appreciable moisture advection begins at all levels, 
coincident with warm advection. Guidance is a little slower to 
move the high out than previous cycles so trimmed PoPs a bit 
Saturday morning-afternoon this package. Anyway expect another 
overunning scenario to set up with widespread moderate 
precipitation moving into the area from S-N Saturday night into 
Sunday morning. Unlike Wednesday's over-running setup there 
should not be much cold air in place to start the event so 
expect precipitation type to be rain over more or less the 
entire area, unless the precipitation starts earlier in the day 
on Saturday then currently expected. 

With PWAT values over 1.3 inches by Sunday morning coupled with
dynamic lift from a shortwave energy propagating ahead of the
Upper Midwest shortwave expect decent QPF with the weekend
system. Although current QPF values only come out to around an 
inch for the event things, hydro concerns will still need to be

Sunday afternoon-Monday: The parent sfc.low will pass through
the Great Lakes on Sunday with a mostly dry cold front passing
through the area Sunday afternoon. It should be noted highs in
the warm sector ahead of the front will likely exceed 60 over
most of the area outside of far northwestern areas. This 
surface low is notably deeper than the last several lows which 
have traversed northwest of the area (modeled at around 970 mb 
by Sunday afternoon), consequently a rather substantial pressure
gradient will establish itself following fropa. Additionally 
the upper level system is much better organized so winds aloft 
increase considerably as well on Sunday with over 100 knots of 
flow at 500, and over 50 knots at 850. Given substantial cold 
advection aloft promoting mixing, there will be the potential 
for very strong (> 50mph) wind gusts both Sunday and Monday 
afternoons. Currently went on the higher side of Wind Gust 
guidance for this period but didn't go as high as straight
mixing of 850-925 momentum would allow (primarily since the 
event is a few days out and is beyond the temporal range of 
higher-resolution guidance). Seasonable to slightly below 
normal high temperatures are expected Monday:

Tuesday-Wednesday: Tuesday currently looks to be another quiet 
and seasonable day with high pressure moving overhead. By 
Wednesday the pattern change mentioned in the Big Picture 
section starts to come underway and guidance gets out of phase 
although at least some level of activity towards the middle of 
next week is hinted at by all global models.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, 
KILG, KMIV, KACY and surrounding areas.

To 12z...IFR or lower across most terminals most of the time as
low stratus and areas of mist and fog continue. 

Today...IFR/MVFR early then improvement back to VFR as drier 
air arrives. This should occur by the late morning. West winds
10-15 knots with gusts up to 20 knots. 

Tonight...Mainly VFR with just some mid and high clouds around.
NW winds around 5 knots. 


Friday...VFR light northerly/northwesterly winds. High

Saturday...VFR with light winds early. Increasing cloud cover
from SW-NE into Saturday PM. Rain moving in from SW-NE in the
afternoon with most terminals down to MVFR or lower by Saturday
evening. Medium confidence in precipitation timing. 

Sunday...Sub-VFR conditions in rain Sunday morning with SE
winds. Conditions improving behind cold front Sunday afternoon,
with winds veering shifting to the west and increasing
considerably with gusts over 30 kts possible. Medium confidence

Monday...VFR with NW winds 20-30 kt with gusts over 45 knots 
possible. Low confidence in max wind gusts.


Marginal SCA conditions continue through today for the coastal
waters as west winds gust 20-25 knots and seas hover around 5 
feet.On the Delaware Bay, winds and waves should stay below SCA

Conditions should be sub SCA everywhere for tonight as seas
subside below 5 feet with NW winds around 15 gusting up to 20


Friday through Friday night...Sub-SCA conditions expected.

Saturday through Saturday night... SUb-SCA conditions with VSBY
restrictions in rain and possible fog Saturday night.

Sunday...Borderline SCA conditions early Sunday in easterly
flow, however much stronger winds Sunday afternoon and night
with westerly winds 20-30 kt with 35-40 kt gusts.

Monday...NW winds 25-30 kt with 35-50 kt gusts possible.


With onshore flow through this morning, water levels will run a
bit high around the high tide later this morning. Winds shift 
to the west after sunrise, and this should help mitigate water 
levels from rising to Advisory thresholds. Tide levels may run 
from 1/2 foot to as much as 1 foot above normal during the high 
tides today, and a few spots, mainly along the Atlantic coast, 
may briefly touch Advisory thresholds. No Coastal Flood 
Advisory at this time as it will not be widespread.


MARINE...Small Craft Advisory until 6 PM EST this evening for 


Near Term...Fitzsimmons/Kruzdlo
Short Term...Fitzsimmons
Long Term...Carr
Tides/Coastal Flooding...