Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
344 PM EST Thu Nov 14 2019
Valid 12Z Sun Nov 17 2019 - 12Z Thu Nov 21 2019
...Pattern Overview and Guidance/Predictability Assessment...
Latest guidance expects a fairly amplified western ridge/eastern
trough pattern aloft into the first half of next week. Pacific
energy moving into western North America from Tue onward should
eventually eject the eastern trough. The primary forecast
problems involve specifics of two possible western Atlantic
systems associated with energy in the eastern mean trough, and the
ultimate evolution of energy that arrives at the West Coast around
Tue-Wed.
Over the East the majority of guidance shows one system off the
southern Mid-Atlantic coast on Sun lifting north-northeast as
upstream troughing approaches. The 00Z CMC strayed to the
slow/western side of the envelope by day 5 Tue though a number of
CMC ensembles were also westward but faster like remaining
guidance. There is a decent signal that the eastern trough energy
will generate another area of low pressure off the East Coast by
Tue-Wed. Especially by late Wed the 00Z ECMWF was among the most
western solutions. Confidence in specifics is low given various
model/ensemble ideas for whether upper energy will close off and
where an embedded closed low will track. In both cases prefer the
best model-ensemble consensus that maintains reasonable continuity.
For the energy nearing the West Coast by Tue there are already
meaningful differences in timing and whether there will be a
closed low within the upper trough, with the scale sufficiently
small to lower predictability several days out in time. Toward
Wed-Thu the most prominent difference is that GFS runs have tended
to be more progressive with the southern part of the trough versus
the ECMWF/CMC and their ensemble means as well as other
operational models. The 06Z GEFS mean has also adjusted somewhat
toward the ECMWF cluster's idea. This is connected to the GFS
being fastest to bring height falls into the upstream ridge. The
majority consensus of guidance and recent difficulties that GFS
runs have sometimes had in properly amplifying energy into closed
lows would recommend leaning toward the ECMWF cluster by late in
the forecast.
Based on the above considerations the updated forecast
incorporated ideas from the 00Z GFS/CMC and past two ECMWF runs
for about the first half of the period and then trended toward a
blend among the ECMWF runs/00Z CMC and 06Z GEFS/00Z ECMWF means.
This helped to downplay lower confidence details late in the
period.
...Weather Highlights/Threats...
Expect chilly weather over the East into the start of next week,
with coolest readings of 10-20F below normal likely on Sun.
Temperatures should moderate toward normal by the middle of next
week. On the other hand areas from the West into parts of the
northern-central Plains will see well above normal temperatures
(up to plus 10-20F anomalies) especially Sun-Mon and to some
extent Tue, followed by mostly single-digit anomalies above or
below normal.
The lead system off the Mid-Atlantic coast should keep the
heaviest rains offshore but could be close enough to produce some
rain along the coast. If any moisture extends farther inland it
is most likely to be over the Northeast. In that case the western
periphery of the moisture shield could contain some snow. The
strong pressure gradient between the low and the surface high
exiting New England will lead to windy conditions along the coast
and especially over the nearshore waters. Please see the Ocean
Prediction Center's products for more information. Precipitation
coverage and intensity with the second area of low pressure
evolving Tue-Thu are highly uncertain, with the Northeast again
the most likely area to see some rain and inland/higher elevation
snow if the moisture shield extends far enough westward.
Favored terrain in the Pacific Northwest/northern Rockies will see
one brief episode of light-moderate precipitation Sun-Sun night.
Another area of moisture should cross the region around late Mon
through Tue or so with similar or slightly higher totals. The
majority scenario of an upper low settling over or near southern
California mid-late week would increase the potential for
significant precipitation (mostly rain) over the Southwest and in
particular central Arizona. Still, guidance spread is
sufficiently great to temper confidence regarding precip
coverage/intensity. Remaining western energy aloft progressing
across the northern half of the lower 48 may bring a front and
accompanying precipitation into the east-central U.S. by next Thu.
Rausch
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy rain across portions of the Southwest, Wed-Thu, Nov 20-Nov
21.
- Flooding possible across portions of the Northern Plains.
- Flooding occurring or imminent across portions of the Northern
Plains.
- High winds across portions of the Mid-Atlantic and the
Northeast, Sun-Mon, Nov 17-Nov 18.
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml