Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Sun Oct 27 2019
Valid 12Z Wed Oct 30 2019 - 12Z Sun Nov 03 2019
...Much below normal temperatures over the West/Plains as of Wed
will gradually moderate...
...Developing storm likely to affect the eastern half of the
country during the latter half of the week...
...Overview and Guidance/Uncertainty Assessment...
Guidance still shows some broadening of the eastern Pacific mean
ridge aloft over the course of the period, ultimately leading to a
neutrally tilted mean trough settling into the east-central U.S.
in contrast to the positively tilted trough extending into the
West early-mid week. While some spread persists for the
specifics, there is a steadily improving signal that the last
significant bundle of energy aloft ejecting from the West will
generate a deepening storm system around Thu-Fri. The surface low
should track from the Mississippi Valley/Ohio Valley
north-northeast through the Great Lakes into southern Canada and
be accompanied by a broad shield of various precipitation types.
Depending on how much the storm deepens, strong winds will also be
possible as it reaches/passes the Great Lakes. Expect much of the
lower 48 to be fairly dry after this storm departs.
Maintaining fairly good continuity, the updated forecast
emphasized the 12Z ECMWF/UKMET/CMC during the first half of the
period. Then the forecast steadily increased 12Z ECMWF mean input
toward half weight while holding onto some aspects of the 12Z
ECMWF/CMC (plus adding the prior 00Z ECMWF run to account for
increasing detail uncertainty).
The GFS has been trending slower with the upper low ejecting from
the central Rockies but continues to be on the faster side of the
spread. Recent verification and trends have steadily favored the
slower majority cluster of guidance. Interestingly the past
couple GEFS mean runs have aligned quite well with the ECMWF
cluster aloft for Wed-Thu but become more open than consensus
thereafter. The new 00Z CMC evolves to an open wave aloft as well
but remaining models maintain an upper low and in fact become
deeper and more concentrated as the system reaches the Great Lakes
by Fri. However a weaker trend in recent ECMWF runs does temper
confidence in the deepest GFS/UKMET solutions.
Behind this system there is decent agreement that cyclonic flow
aloft will bring one cold front into the northern tier by Fri
(weakening by Sat) with another front approaching by Sun. By next
weekend guidance differences are fairly modest outside of details
that have low predictability at the days 6-7 time frame,
supporting a model/ensemble mean blend. Recent GEFS means have
tended to be on the weaker side of the spread for the eastern
Pacific ridge aloft so that solution was excluded from the blend.
The new 00Z GEFS mean seems to have trended in a favorable
direction though.
...Weather Highlights/Threats...
The dominant weather focus during the period will be the upper low
expected to track from the central Rockies through the Great Lakes
and into Canada, with deepening low pressure lifting through the
east-central U.S. Thu-Fri. The best potential for meaningful snow
in the cold sector should extend from the central Rockies/Plains
northeast into the Upper Great Lakes. Ahead of the storm expect a
broad area of rainfall and possibly considerable coverage of heavy
amounts. Activity may be enhanced by an area of moisture
currently over the Caribbean and expected to lift northward across
the Gulf of Mexico into the southern states. Some embedded
convection could be strong as well. There may be a period of
brisk to strong winds over the Great Lakes/Northeast as the
deepening surface low reaches and departs from the Great Lakes.
Behind this system expect much of the lower 48 to be dry, aside
from some lake effect rain/snow and scattered light precipitation
with fronts affecting the northern tier as well as mostly light
rainfall along the front settling over the Florida Peninsula.
The central Rockies/Plains will be in the core of coldest air
around midweek with some locations seeing highs 30-40F below
normal. A much broader area from the Interior West through
central U.S. will see temperatures at least 10-20F below normal.
Some daily records for lows/cold highs are likely. Readings will
trend warmer from late week into the weekend over the West and
into the Plains, with the eastern Pacific upper ridge possibly
nudging far enough eastward to bring modestly above normal
temperatures to the West Coast states by the weekend. Meanwhile
the East will see a cooling trend by the weekend after warm/moist
flow leads to a couple days or so of plus 10-20F anomalies for
morning lows mid-late week. The cool anomalies over the East by
the the weekend will be much less extreme than those forecast
farther west early in the period.
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
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