Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Mount Holly New Jersey
1252 PM EST Wednesday Dec 18 2013
a high pressure system in the Tennessee Valley will move east and
pass off the North Carolina coast on Thursday. A cold front should
reach the Great Lakes on Friday and slowly sag and then stall over
or just north of the forecast area on the weekend as a low pressure
system develops along it in the Southern Plains. After this low
pressure system reaches New England on Monday, it will push a cold
front through the area. Yet another Canadian high pressure system
will build in behind the front and its expected to reach the
Appalachians on Tuesday.
Near term /through tonight/...
1230pm estf update sent at 1150 am: raised temperatures a tad today from
the prior forecast...added 4 knots of wind (25kt gusts) and lowered skycover
to a sunny day most areas. A nice Winters day with temperatures about 5 degrees
A fb image of the snow cover has been posted on The Mount Holly
web site...includes last nights one half to 1.5 inch snow on Delaware-Maryland-Virginia
to Cape May New Jersey and the deeper 5 to 15 inch pack northwest of Interstate
Tonight...some thin cirrus at times but thin enough to permit
radiational cooling despite wind turning light SW. Guidance has
been much too warm over the thick snowpack of NE New Jersey and portions of east
PA the last two nights...in some cases 10 to 15 degrees too warm. The
European model (ecmwf) 2m temperatures were much much better than NCEP guidance and so
will watch for the 12z/18 European model (ecmwf) 2m temperature guidance arriving at 1pm
and adjust accordingly. Already have lowered mins there about 5
degrees below the coldest of the 12z/18 NCEP MOS guidance and
still think that is too warm for the Sussex County to Somerville
areas as well west to kabe and krdg. This is much much closer to
the 00z/18 European model (ecmwf) MOS temperature temperature guidance....close to 10 above in
Sussex County et cetera. If snowcover remains over S Delaware late today...there
could be additional radiational cooling beyond what we are forecasting
Short term /Thursday/...
dry warm air advection with SW wind gusting to 15 knots in the
afternoon...thick cirrus at times otherwise p to M/sunny. In these
situations...the MOS guidance may be linked too much to the
cooler skin temperature of the snowcover and have chosen to forecast the
warmest of available MOS guidance in the 95 corridor and a 50 50 blend
of the guidance over the snow cover of northwest New Jersey and NE PA.
Long term /Thursday night through Tuesday/...
***record warmth potential sunday***
The GFS initialization and short term verification was much better
than last night. Both the GFS and WRF-nmmb were a bit too cold at
850mb off their initialization, but pretty good at 925mb. As per
pmdhmd, we are going to use a model consensus blend in the long
Giving continued meaning to the expression there is no such thing
as a free lunch, the December thaw that is expected to occur this
weekend may come with a price as snow melt and rain might combine to
produce flooding. After that we are seeing dry weather and a return
of seasonable temperatures heading toward Christmas.
On Thursday night, the models and WRF-nmmb in particular develop
either stratus or fog in the southwest flow regime. Have to go back
three years to try to recall a similar scenario and that train left
the yard a long time ago. Given the winds, even with the snow
cover, it looks as though the WRF-nmmb is too bullish with forming
low clouds (although it does have some sref support) in the snow
cover areas. So we hedged it partly cloudy. We expect enough of a
lull with the long long night for min temperatures to outperform and we
sided toward the colder stat guidance.
The operational runs of the GFS and European model (ecmwf) are close and close to
the European model (ecmwf) ensemble mean. The GFS ensemble mean sags the front
into our County Warning Area on Saturday. There is room for some temperature
busts if the front does sag farther south. Nonetheless the cold
front should not be a geographical problem on Friday. Maximum temperatures in
the snow covered areas was kept down, while we sided with stat
guidance elsewhere. Given the exit region of the 250mb jet is well
situated, even with the slower ECMWF, we maintained afternoon probability of precipitation
Temperatures and to a lesser extent the weather becomes complicated
Friday night and Saturday depending on where the front stalls. Right
now its expected to remain just north of our County Warning Area. Probability of precipitation were
emphasized north. We double hedged because of the front and
remaining snow to have conservatively lower mins and maxes north.
Aloft its warm enough for just rain. Have to watch far far north
about freezing rain chances Friday night. Op models bring surface
temperatures to the middle 30s far far north, similar to stat guidance.
If there is a cold wiggle for Saturday, all of the medium range
models are bullish about a very warm for December on Sunday. We did
not go as high as their stat guidance, our mins and maximum temperatures are
still about 20 to 25 degrees higher than normal. Here comes the
lack of a free lunch as the unseasonably warm weather should give
the snow pack a major hit. Coupled with an unseasonably moist air
mass, gfs's predicted precipitable waters are beyond the 99th percentile for
December, comes the flooding potential. Marfc ensemble guidance is
already showing some flooding chances in our area, centered around
the Raritan basin.
The GFS remains inexplicably light with its forecast quantitative precipitation forecast in spite of
the forecast precipitable waters . Here the can ggem and European model (ecmwf) at least paint the
potential picture better. The long wave trough is not forecast to
become negatively tilted, but remain positively tilted, part of
the reason for the subdued quantitative precipitation forecast. Too far in the future to be
confident that this trough feature will verify. Lastly, while not on
the level of the pair of ncfcb we had in November, the forecast 925mb
and 850mb winds reaching 60-70kts starts touching the
uncomfortable level for wind related headlines. Right now upright
convective parameters remain southwest of our County Warning Area.
Our gridded forecast maximum temperatures for Sunday are generally within 2
degrees of record and or exceeding the previous record. The 00z/18
European model (ecmwf) mass field information converted to NCEP MOS the past two days has
forecast Georgetown Delaware on Sunday to reach the upper 70s. If its
partly sunny with a breezy southwest wind...that excessive pre
Christmas warmth will occur. However...we're only forecasting around 70
at this time due to uncertainties of skycover...wind direction.
We admit we are dragging our probability of precipitation on Monday and Monday evening. We
opted for an ec and GFS timing blend as they are in their respective
timing camps. Most if not all of the precipitation is forecast to shut off
before a potential change to wintry ptype. Low confidence about
temperatures Monday because of the cold frontal timing. Then Tuesday
more confidently should be dry with a return of seasonably cold
temperatures. Christmas day also appears dry.
Aviation /18z Wednesday through Monday/...
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg,
kmiv, kacy and surrounding areas.
This afternoon...VFR scattered to briefly broken clouds at or above 4000 feet with
the broken skycover restricted to kabe and krdg. West-northwest wind gusts to
near 25 knots.
Tonight...VFR with some cirrus late. Wind becomes light west or SW
during the night.
Thursday...VFR with varying amts of cirrus at or above 20000 feet. Wind SW
gusts around 15 knots in the afternoon.
Thursday night...some MVFR conditions possible northwestern airports.
Mainly VFR elsewhere.
Friday...mostly VFR. Chance of MVFR conditions northwest. Some
gusty southwest winds possible on the coastal plain airports.
Friday night and Saturday...mainly VFR. Some MVFR conditions in
showers with a greater chance/confidence northern airports.
Saturday night and Sunday...predominately MVFR. IFR at times in low
clouds and precipitation. Strong gusty southwest winds potentially exceeding
30 kts with low level wind shear conditions possible before winds start increasing.
west-northwest wind Small Craft Advisory conditions should temporarily end around 23z. Then
resume as a SW wind Small Craft Advisory Thursday midday....esp since the bl temperature
still will be below the SST of 6c-10c.
Thursday night through Saturday...an increasing return southwest
flow. There is a chance for Small Craft Advisory conditions to
develop on Thursday again and continue into Saturday. The limiting
factor may be that the air mass will be becoming warmer than the
ocean and Bay waters.
Saturday night and Sunday...the southwest flow is forecast to
intensify even further preceding the cold front bringing the
potential for gale force wind gusts by Sunday.
daily record maximum temperatures that are vulnerable to be exceeded.
12/21 kged 65-1956 por back to 1949
Kacy 65-1998 por back to 1874
kphl 64-1998 por back to 1872
kilg 64-1998, 1984 por back to 1894
kabe 61-1998 por back to 1922
kttn 64-1998 por back to 1865
kged 68-1949 por back to 1949
krdg 61-1998, 1949 por back to 1869
kmpo 58-1990 por back to 1901
For the month we've taken a look at the records and for the time
being we will only list the vulnerable short period of record
kged 77 set 12/1991. The monthly records for all our County Warning Area long term
climate stations are in the 70s and in most cases featured the
period of 12/4-7 1998. The mpo all time December record is 66.
Marine...Small Craft Advisory until 6 PM EST this evening for anz430-