Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
318 PM EST Fri Mar 01 2019
Valid 12Z Mon Mar 04 2019 - 12Z Fri Mar 08 2019
1500 UTC Update...
Overall model/ensemble consensus remains above average through
much of the medium range, with general agreement at the large
scale even through day 7. Clustering of solutions was sufficient
both with respect to the low pressure system off New England on
day 3 (Mon) and a low pressure system approaching California on
day 5 (Wed) that a multi-model deterministic blend (including the
ECMWF/UKMET/CMC/GFS/FV3) served as a good forecast starting point.
Differences emerge after day 5 with respect to how this West Coast
system evolves as it moves inland, with the GFS (and CMC) keeping
a much more amplified shortwave across the Rockies and central
U.S. days 6-7 (Thu-Fri) relative to the ECMWF/FV3 which quickly
weaken the energy as it moves into broad anticyclonic flow. At
this time ensemble means all support the existence of this broad
anticyclonic flow over the Rockies by that time, so support for
the GFS solutions seems limited. Thus, chose to with a solution
for days 6-7 based on a blend of the ECMWF and majority ensemble
means (ECENS and GEFS). For the most part, adjustments to
continuity were relatively small. The previous discussion from
overnight follows below.
Ryan
Previous Discussion (issued at 0643 UTC)...
...Overview...
A return to a more amplified upper level flow pattern is expected
across the continental U.S. for next week. The passage of a
strong cold front across the eastern U.S. by Monday morning will
herald a much colder airmass across the central and eastern U.S.,
accompanied by a pronounced upper level trough and sprawling
surface high from Canada. The presence of the eastern U.S./Canada
trough supports a broad upper ridge across the Rockies and High
Plains by midweek which gradually flattens out towards the end of
the forecast period. An unsettled weather pattern is expected to
return across the West Coast region as multiple Pacific
disturbances move inland.
...Model Guidance Evaluation and Preferences...
With the transition to a more amplified pattern over the
continental U.S. compared to the quasi-zonal pattern in the short
range period, there has been an improvement in model agreement
over the past few model cycles regarding the medium range forecast
period next week. This particularly pertains to the large scale
upper level trough anchored over the eastern half of the nation
through Thursday, although the 12Z ECMWF indicates a faster
shortwave progression across the eastern U.S. on Tuesday compared
to the other guidance.
The main forecast challenge resides over the West Coast region
beginning on Wednesday with the approach of a strong Pacific storm
system towards California. The 12Z ECMWF and the EC mean are
suggesting a more northern track with the surface low, and the GFS
and CMC are indicating a southerly track, with the GEFS mean close
to the model consensus. There are indications in the
deterministic guidance and many ensemble members that a Rex block
may try to develop over the northeast Pacific and western Canada
as an upper ridge tries to build north of the West Coast upper
trough. This is where the greatest model differences currently
exist during the second half of the forecast period, and the
ensemble means provide a more reasonable idea on placement of main
weather features.
The forecast was primarily derived with a blend of the latest
deterministic solutions (ECMWF/GFS/CMC/UKMET) as a starting point
for the Monday through Wednesday time period. For Thursday and
Friday, the GEFS and EC means along with smaller percentages of
the aforementioned deterministic model were incorporated into the
forecast process. No major changes from previous WPC continuity
were warranted.
...Sensible Weather and Potential Hazards...
The winter storm across the East Coast Monday morning will be
intensifying as it continues tracking towards the northeast and
exiting New England later that day. Heavy snow is likely to be
ongoing across interior portions of New England and rain for
coastal areas, with most of the precipitation ending prior to 12Z
Monday. Behind this nor'easter will be a strong cold front that
will usher in an anomalously cold air mass by early March
standards. A huge arctic surface high will encompass all of the
central and eastern U.S. through the middle of the week, and
reaching well into Mexico. The projected departures from early
March climatology are astounding, with high temperatures on Monday
on the order of 35 to 40 degrees below normal across much of the
central and northern plains, and extending eastward across the
Upper Midwest! This equates to subzero lows from Montana to Lake
Michigan, and highs in the single digits and low teens. There
will be a high likelihood for numerous daily record lows to be
established with this arctic airmass. Temperatures only slowly
moderate through Tuesday and Wednesday with anomalies of 20 to 30
degrees below normal, and this will extend eastward towards the
Appalachians. Cold weather will continue through the remainder of
the week across these areas.
Heavy rain and mountain snow are expected to make a return to the
western U.S. by Tuesday night as an atmospheric river event
accompanies a strong Pacific low headed towards California. The
Sierra Nevada will likely get hammered by very heavy snow, thus
adding to its extensive snow pack. Significant snow is forecast
for the central and northern Rockies by Wednesday and Thursday as
the moisture plume advects inland, albeit not to the same degree
as California. Heavy rainfall across the lower elevations of
California may lead to some flooding problems, and future
excessive rainfall outlooks will have additional information as
this event enters the short range forecast period.
D. Hamrick
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
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