Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 AM EST Mon Feb 25 2019
Valid 12Z Thu Feb 28 2019 - 12Z Mon Mar 04 2019
...Pattern Overview...
For the period as a whole expect progressive and broadly cyclonic
mean flow aloft over the Lower 48. As an upper ridge initially
extending from the Gulf of Alaska into the Arctic drifts into
northwestern Canada, a deep south-central Canada upper low will
descend close to or across the Upper Great Lakes region during the
weekend and then continue rapidly eastward. Flow around this low
will likely interact with energy from a Pacific Northwest coast
upper low that opens up as it ejects inland. The combination of
these features should produce an Ohio Valley/Great Lakes into
Northeast storm system by late Fri or Fri night into the weekend.
Meanwhile another Pacific system will likely reach the West Coast
by the weekend. Anticipate highest precipitation totals over
favored terrain from California into the central Rockies and from
the Gulf Coast region into the Northeast, while much below normal
temperatures persist over portions of the central U.S.
...Guidance Evaluation/Preferences...
There are still some details that remain to be resolved but in
principle the guidance is gradually improving in its clustering
compared to some previous days. There is good agreement on the
overall evolution of ejecting Pacific Northwest upper low energy,
the southern Canada upper low that may brush the Upper Great
Lakes, and their interaction that should produce a storm system
over the northeastern quadrant of the country by late Fri into the
weekend. However medium-smaller scale detail and track/timing
issues (which have lower predictability) lead to ongoing
uncertainty for the precise character of the storm system. GFS
and some GEFS runs have been a bit on the faster and amplified
side of the envelope with the ejecting western energy, tending to
result in some of the stronger solutions for the downstream
surface low. ECMWF runs have been inconsistent while the past
couple CMC runs have been fairly weak. The 00Z UKMET is much
weaker than its prior run. Continuity provides fairly high
confidence in the storm's existence and to some degree the track.
However dependence of the storm's evolution on lower
predictability subtle details as well as the observed
inconsistency/spread of operational model runs for depth favor an
intermediate solution for a single deterministic forecast.
For the system forecast to approach California by the start of the
weekend, solutions have maintained moderate spread for strength
and track--northern solutions tending to adjust back down toward
majority clustering over subsequent runs. On the other hand there
has been a persistent spread for timing. The CMC has backed off a
bit from its 12Z run but remains in the faster portion of the
spread (along with recent UKMET runs) while the past couple of
ECMWF runs are slower than earlier runs and now cluster more with
GFS/GEFS runs that have generally been in the slower half of the
spectrum. Behavior of guidance over recent days favors
maintaining a compromise approach as the best starting point.
Guidance agrees that supporting energy aloft will rapidly shear
out to the east as it becomes embedded within the progressive mean
flow.
Based on system considerations the updated forecast started with
an operational model blend for days 3-4 Thu-Fri and then adjusted
toward a blend of 18Z GFS/12Z ECMWF runs and their ensemble means.
...Weather Highlights/Hazards...
During Thu-Fri the initial flow of moisture over the West will
bring areas of rain and higher elevation snow to about the
northern 2/3 of the region with a brief drying trend progressing
from west to east. The next Pacific system should start to bring
moisture into California by the start of the weekend with
rain/mountain snow then spreading eastward into the Rockies. For
the 5-day period expect an area of enhanced precipitation from
northern-central California/southwest Oregon into the
north-central/central Rockies with highest totals likely over
windward terrain of the Sierra Nevada range.
Late this week diffuse shortwave energy may assist in producing
rainfall of varying intensity over the southern half of the East
while the northern fringe of the moisture shield could contain a
little light snow from the Mid Mississippi Valley into the
northern Mid-Atlantic. The developing storm system expected to
reach the Northeast during the weekend should initially spread
snow across parts of the northern Plains late this week with
precipitation coverage then expanding over the eastern states.
Best snowfall potential will extend from the Midwest through the
Great Lakes into New England. Pockets of locally moderate-heavy
rainfall may be possible over the South but thus far with no
coherent signal in the guidance for precise location/magnitude.
This system may produce strong winds over some areas even if not
to the magnitude of the system currently affecting the Great
Lakes/Northeast. Expect a period of lake effect snow in the wake
of this system while lingering moisture along southern latitudes
may produce some rain near the Gulf Coast into early next week.
Behind the aforementioned developing storm, a strong surge of very
cold air will spread south and east across the central/eastern
U.S. from late week onward. The forecast has been consistent in
focusing coldest anomalies (at least 30-40F below normal) over the
northern Plains while high temperatures of at least 10F below
normal may reach the Gulf/East coasts by next Mon. Expect the
best potential for daily records to be for cold daytime highs over
the northern-central Plains and Midwest during the weekend. The
mean pattern will tend to keep the southern 2/3 of the West warmer
than average for morning lows but near to slightly below normal
for highs. Northern areas should remain below normal. The South
and parts of the East will see above normal temperatures late
week/early weekend ahead of the developing storm system before the
trend toward colder readings.
Rausch
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation and winter weather outlook
probabilities are found 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