Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
752 am EDT Sat Jul 26 2014
a warm front will move north across the Ohio Valley and middle-Atlantic
today and tonight. This warm front will then push north of the
region early on Sunday. A cold frontal boundary with an associated
low pressure system then will sweep from west to east across the
region on Monday. This will be followed by a high pressure system
which will build into and pass through the region for the middle and
later half of next week.
Near term /until 6 PM this evening/...
the trend of the strong short wave both in modeling on radar has become
wetter. We are doing an early update to expand the mention of
showers eastward and also carrying higher probability of precipitation.
High pressure will move to the east today, but we should still have a dry or
mainly dry day. An area of low pressure near the Great Lakes will move slowly
eastward and its associated warm front will move through late.
As the warm front approaches, there could be some shwrs/tsra. The guidance
suggests the best chances are S prior to sunset. Will keep some low
chance probability of precipitation in these locations.
Short term /6 PM this evening through 6 am Sunday/...
the aforementioned area of low pressure will drop southeastward tonight and
its associated cold front will approach from the west. There are some diffs in
the model guidance as we approach the end of the short term period with respect to probability of precipitation.
Overall, the GFS is the wettest model and much wetter toward 12z sun.
The European model (ecmwf) is dry at the beginning of the period and implies some quantitative precipitation forecast toward
the end of the period. The NAM/WRF is supportive of the drier European model (ecmwf) and
the CMC is almost completely dry, leaving the GFS as the wet
outlier. Therefore, will just carry some schc or low chance probability of precipitation into
the overnight, and some low quantitative precipitation forecast as well. There are also some diffs
on the areas that would see any qpf, with some models suggesting S and
others north. The best chances for precipitation on sun will be after daybreak.
Long term /Sunday through Friday/...
sunday: continued west-southwest flow as the warm front pushes
through the region. Two potential areas of convection may be in
progress by this point to start the day. The first is likely to be
located to our northeast in association with a shortwave and the second
over the Ohio Valley where another shortwave coupled with very
favorable jet dynamics are likely to lead to a mesoscale convective system. These areas of
thunderstorms may produce some debris clouds throughout the day
across the region.
The thunderstorm cluster over the Ohio Valley in the morning may
redevelop further east in the afternoon across our southern half
of the region as shown on the 00z hires NAM. Several past GFS runs
have hinted at this as well. Most of the operational models
(ecmwf,GFS,CMC,UKMET 00z runs) and the rgem generate another
cluster of storms over the eastern Ohio Valley Sunday afternoon,
but this should hold off till evening.
850/925 mb temperatures still would translate to a fair shot at 90
for kphl. However based on the potential for some debris clouds
will stay at or just under the met/mav guidance sets on high
temperatures which is back in the middle 80's for many.
In terms of strong to severe storms the latest Storm Prediction Center outlooks
maintain a slight risk for severe storms from Sunday into Monday
with the southern part of the region having the highest chance
even for significant severe. There is quite a spread in terms of
destabilization within the Storm Prediction Center sref plumes, from 700-3000 j/kg
cape at phl for 21z on Sunday. However effective bulk shear of
30-50 knots is well established with the operational models and
the sref. In model solutions where destabilization occurs, lapse
rates steepen enough with the corresponding shear to suggest a
hail threat and the potential for gusty downdraft winds. Precipitable water values
will be close to 2 inches suggesting the potential for locally
heavy downpours as well. Enhanced wording has been added to the
grids after 18z. Used elements from the wpc quantitative precipitation forecast and the 21z sref
for quantitative precipitation forecast. Stayed with low likely probability of precipitation given some uncertainty still
present in the exact evolution with the thunderstorm clusters.
Sunday night: the highest chance for precipitation in will be Sunday
night in association with the strongest vorticity maximum and associated
shortwave which has likely already lead to the development of
another mesoscale convective system to our west. This mesoscale convective system is likely to move east during the
night. Similar conditions to earlier in the day with some
uncertainty in the amount of instability (cloud cover/ previous
storms) with modest shear leading to a continued risk of gusty winds
and hail with thunderstorms. Enhanced wording as well in this
period. Continued low end likely probability of precipitation Sunday night given the
uncertainty in the location of the mcs's. 00z hires NAM implies
main development to our south in this period which added hesitation
to increasing probability of precipitation. For quantitative precipitation forecast used both the wpc quantitative precipitation forecast and the 21z sref.
Stayed close to the met and mav guidance sets for temperatures.
Monday and Monday night: the cold front moves through likely Monday
afternoon. The majority of showers and storms should be east of the
region. However some showers/thunder may linger with the vorticity maximum and
trough axis moving through the region. Highest shower chances across
the north. A gusty day is likely as modeled momentum Transfer
continues to show the potential for some gusts near 20 miles per hour. 850/925
mb temperatures are a bit cooler with recent model runs when compared to
the past few days also morning clouds may be issue as well with
afternoon highs. Knocked afternoon highs down a touch from previous
forecast which matches up well with the met and mav.
Tuesday through friday: another Canadian airmass comes down into the
region with high pressure in control through Thursday. The high
pressure moves east of the region by Friday allowing for some
moderation back to normal. Cool northwesterly flow will be in place
Tuesday and Wednesday as most locations struggle to 80 for highs.
Even with this abnormally cool airmass there is good agreement
among the mex, wpc and modeled two meter temperatures in this
period. Went on the cooler end of guidance particularly the mex,
close to wpc in most periods. Lapse rates may steepen up Tuesday
afternoon for the formation of some cumulus/isolated light shower on
the backside of the trough across the north with the aid of
elevation. Maintained slight pop on Tuesday from previous forecast.
Otherwise, each day looks mostly sunny. Dewpoint depressions look to
large for Wednesday and Thursday for much in the way of afternoon
Aviation /12z Saturday through Wednesday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg,
kilg, kmiv, kacy and surrounding areas.
Overall, VFR conds are expected. High pressure will move eastward. A warm front
will move through late and a cold front will approach from the west. There
will be low pop chances today and tonight. Any showers or thunderstorms and rain could bring
lower ceilings/visibilities but chances are low enough at this time not to mention
in the individual taf sites and there are enough diffs in the model
guidance to lower confidence even more. Wind will generally be southwesterly through the
period. The wind could gust as high as 20 knots during the day today, but will
be less than 10 knots otherwise.
Sunday and Sunday night: MVFR and IFR restrictions in thunderstorms.
Some gusty winds possible as well from the thunderstorms.
Monday: gradually turning VFR. Some southwest/west wind gusts around
20 knots possible Monday afternoon. Wind shifting from southwest to
west in the afternoon.
Monday night through wednesday: VFR.
high pressure will move eastward today. Southwesterly flow will
gradually increase over the waters through tonight and seas will
slowly build as well. However, both wind and waves are expected to
remain below Small Craft Advisory criteria through the near and short term periods and no
marine headlines are anticipated.
Sunday and monday:four feet seas will be possible at
times Sunday and Monday. Think wavewatch may be a touch aggressive
with wave heights. 20 knot wind gusts possible as well.
Monday night through wednesday: seas and winds expected to below Small Craft Advisory
criteria at this time.