Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Thu Apr 08 2021
Valid 12Z Sun Apr 11 2021 - 12Z Thu Apr 15 2021
...Overview and Guidance Evaluation/Predictability Assessment...
Latest guidance continues to show various significant
uncertainties with forecast specifics in a blocky pattern. Upper
ridging over and east of eastern Canada with multiple closed lows
likely to meander to the southwest/south--between the northern
tier states and the Canadian Maritimes--will be a dominant theme
from the start of the period. An eastern Pacific ridge that
builds into Canada ahead of an amplifying North Pacific trough
will add to the blockiness mid-late period, likely supporting some
degree of mean troughing over the western U.S. with a possible
embedded upper low.
As for the details, models have varied with what becomes of energy
within a leading compact Great Lakes upper low Sunday-Monday as
upstream energy (likely another closed low) approaches--ejecting
eastward or becoming incorporated into the flow around the
upstream feature. For the upper low coming in from the northern
Plains, the past couple ECMWF runs have been weaker and more
elongated with the energy than most other guidance. New 00Z
GFS/CMC/UKMET runs are still more consolidated with the upper low.
The 12Z ECMWF uses some of its western elongated energy to
develop a deeper closed low a little west of consensus by day 7
Thursday though. Also late in the period GFS runs become more
progressive than consensus with this upper low. GEFS mean runs
have been holding the upper low back toward the Upper Great Lakes,
closer to the majority scenario, due to greater persistence of the
eastern Canada ridge. Meanwhile solutions stray considerably with
time for the upper low initially near the Canadian Maritimes.
Similar behavior among ensemble members is likely playing a role
in the means becoming a lot weaker than most operational runs by
mid-period. Preference would be for an intermediate solution with
the Maritimes low, keeping it a little east of New England.
Specifics over the West are still uncertain as well. There is
reasonable consensus for mean troughing to develop from early in
the week onward but with a fair amount of spread for depth and
existence of a possible embedded upper low. The GFS had been
mostly on its own for the deep and upper low scenario but recent
CMC runs have been trending in its direction and now the 00Z UKMET
shows a closed low for a time as well. Through the 18Z cycle the
guidance spread was sufficient to favor holding close to
continuity via the ensemble means that have all been depicting an
open trough of more moderate depth.
Based on 12Z/18Z guidance comparisons and continuity, the updated
blend started with an operational model consensus early and then
transitioned to nearly all 18Z GEFS/12Z ECMWF means by days 6-7
Wednesday-Thursday due to various low-confidence aspects of
individual models. The 12Z ECMWF was just close enough in
principle to maintain modest input late in the period. Manual
editing improved definition of the Maritimes low at that time.
The new 00Z ECMWF has completely changed to the GFS cluster in
many respects over and near the lower 48, so depending on how the
00Z ECMWF/CMC means trend there could be a significant adjustment
in the most likely scenario.
...Sensible Weather and Potential Hazards...
The leading system affecting the East on Sunday will produce areas
of rainfall over parts of the Great Lakes/Northeast while the
trailing front should support some moderate to heavy rain over
Florida. The upstream system emerging from the West will spread
precipitation across the northern tier, with some snow possible
over extreme northern areas. The combination of the second
frontal system as well as the leading one that stalls over the
northern Gulf/Gulf Coast region could produce some areas of
enhanced rain/thunderstorms from southern portions of the Plains
to the Lower Mississippi Valley/Gulf Coast. Initially light and
scattered precipitation over the northern-central Rockies should
become somewhat heavier and more organized into the central
Rockies from Tuesday onward as upper troughing develops over the
West and high pressure building southward through the Plains
promotes upslope flow over some areas. The best potential for
meaningful snow will be over the southern Wyoming into Colorado
Rockies and possibly east into portions of the High Plains. With
greater uncertainty, some precipitation could extend farther
westward.
Expect portions of the Mid-Atlantic and eastern Great
Lakes/Northeast to see well above normal temperatures on Sunday,
especially for morning lows with plus 15-25F anomalies possibly
near daily record values if they hold for the calendar day. The
Sunday frontal passage over the East will bring temperatures down
only to near or modestly above normal levels early next week. A
cooler airmass will extend from the northern Rockies through much
of the Plains from Sunday into midweek behind another front. Most
days should feature some areas with highs 10-20F below normal.
This cool air will continue into the Mississippi Valley and
portions of the East late in the period but in modified form,
yielding only single-digit negative anomalies. Above normal
temperatures over the Southwest on Sunday should gradually trend
closer to normal as upper troughing develops over the Interior
West. On the other hand an approaching Pacific ridge aloft may
raise northern California and Pacific Northwest highs up to 10-20F
above normal from Monday or Tuesday through Thursday. There are
still enough uncertainties with specifics of flow aloft late in
the period to lower confidence in temperature specifics over the
West.
Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook 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