Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
100 PM EDT Sat Mar 23 2019
Valid 12Z Tue Mar 26 2019 - 12Z Sat Mar 30 2019
...Overview...
An upper low in the eastern Pacific on day 3/Tue will approach the
western CONUS on day 4/Wed, with troughing traversing the
western/central U.S. days 5-7. At the surface, fronts in the West
will help create heavy precipitation early in the period, and low
development in the Plains and movement into the MS Valley/Midwest
could cause heavy precipitation in those areas by day 6-7, but
uncertainty remains high.
...Guidance Assessment/Preferences...
For days 3-4, a multi-model blend worked well for the large-scale
features while de-emphasizing the small-scale differences in the
forecast--models were in general agreement for the upper low in
the eastern Pacific moving eastward into the CONUS. By day 4-5,
the 00Z CMC became much deeper with the trough than other models,
and sent upper-level energy around the upper high in the Gulf of
Alaska in a manner that seemed unlikely, so did not utilize the
CMC for this forecast. Additionally, the operational 00Z ECMWF
began exhibiting differences from its mean and other models at day
5, moving the large upper low in central Canada eastward more
quickly. Moving into day 6-7, this meant that the trough in the
CONUS was cut off from the northern stream flow, and thus was
moving eastward more slowly and much stronger compared to other
models, which affected surface low placement as well. There is
plenty of spread in the ensembles in the degree of separation of
these features and other aspects of the CONUS trough's evolution,
but preferred to stay away from the deterministic ECMWF solution
for now in favor of the means. Used the GEFS and EC means at day 7
with some WPC continuity, with a slight emphasis on the GEFS mean
due to how it handled the transitioning pattern in Alaska.
...Weather Highlights and Hazards...
With the upper low approaching the West, a couple of fronts will
make their way through the West, and heavy precipitation is
expected for day 3-5. Heavy valley rain/mountain snows will
threaten northern California/Oregon through the Northern Rockies,
and heavy snow is forecast for the Central Rockies as well later
in the week. As the trough energy ejects out through the West, it
is expected to induce cyclo/frontogenesis in the Plains, and this
plus another cold front dropping southward out of Canada, as well
as deepening return flow from the Gulf of Mexico, would all fuel
an expanding risk of enhancing rains/convection. The emerging
heavy rain area threatens areas of the Mississippi Valley/Midwest
(where flooding is ongoing) heading into next weekend. Uncertainty
remains regarding heaviest rainfall amounts/placement, but the
guidance signal for development continues to grow. In the cold air
behind the aforementioned cold front coming from Canada,
late-season snow is possible in the Northern/Central Plains to
Midwest.
The cold front moving quickly southward through the Plains and
Mississippi Valley is expected to change the ongoing regime of
above average temperatures there in the short/early medium range
period to below normal by day 6-7 (Fri-Sat). Ahead of this front,
the Eastern Seaboard is expected to be warmer than average by Sat
after the cold high pressure dominates midweek.
Tate
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