Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Mon Oct 28 2019
Valid 12Z Thu Oct 31 2019 - 12Z Mon Nov 04 2019
...Much below normal temperatures over the Rockies/Plains will
gradually moderate...
...Developing storm likely to affect the eastern U.S. Thu-Fri...
...Overview and Guidance/Uncertainty Assessment...
Models and ensemble means agree that a persistent eastern Pacific
ridge will promote broad cyclonic flow downstream, with a mean
trough axis tending to align over the east-central states.
Guidance consensus and continuity provide fairly high confidence
in the large scale pattern but individual embedded features have
lower predictability. By early next week there are also some
aspects to the forecast that become more uncertain near the West
Coast and into western Canada.
In terms of sensible weather effects the greatest forecast problem
continues to involve the upper trough expected to be over the
central U.S. as of early Thu and eject northeastward thereafter,
supporting development of surface low pressure that tracks from
near the Ohio Valley northeastward into eastern Canada. Over the
past day there has been some notable trending in the guidance as a
whole toward faster progression (likely due in part to a stronger
trailing upper trough dropping southeastward from Canada) and a
weakening of the surface low among the prior strongest solutions
(some ECMWF/GFS/UKMET runs). The past couple CMC runs are in the
minority with a more phased/open system aloft leading a faster and
less wound-up wave. Spread and trends in the guidance as well
less than usual clustering among individual ensemble members for
the time frame involved keep confidence in any specific solution
lower than average. Based on guidance available through the 18Z
cycle the updated forecast was based on a blend among the 12Z and
00Z/27 ECMWF runs along with the 18Z GFS and 12Z UKMET. This
solution kept GFS weight low enough to yield minimal influence on
day 3 Thu when that model is farther south and/or east than most
other guidance aloft. After that time the GFS fell more in line,
and in fact was slower than the ECMWF/ECMWF mean by early Fri. By
Fri some of the new 00Z guidance continues to trim away the
western/slower side of the prior model envelope for the surface
low--but in a significant exception the 00Z ECMWF has adjusted
slower. At that time the manual forecast falls between the new
and previous ECMWF runs.
Behind the eastern system the aforementioned model blend
represented consensus well for the upstream trough aloft
amplifying into the lower 48 along with the associated cold front
which should weaken by Sat. Given the time frame involved there
is decent agreement and consistency with the next frontal system
expected to reach the northern tier/Great Lakes by days 6-7
Sun-Mon. The scale and progression of shortwave energy aloft
suggest there could be more variability in future runs though.
Late in the period GFS/GEFS mean runs develop lower heights aloft
near/offshore California than most other guidance. In addition
the GEFS mean remains among the weakest guidance with the Pacific
ridge and the 12-18Z GFS runs gradually shifted the ridge-trough
pattern a little east of consensus (while the 00Z GFS looks better
in that regard). As a result the forecast adjusted toward primary
emphasis of recent ECMWF runs and the 12Z ECMWF mean. The 18Z
GEFS mean did compare acceptably for the overall North America
trough to merit a small minority weight in the forecast.
...Weather Highlights/Threats...
Confidence remains lower than desired for exact details of the
developing system likely to track from the Ohio Valley through the
eastern Great Lakes and into eastern Canada during Thu-Fri.
However there is still a common theme of a widespread
precipitation shield across the eastern U.S. with some snow
possible in the northwestern periphery of the system (Midwest to
Upper Great Lakes) and areas of heavy rainfall along with pockets
of strong convection in the warm sector. Currently expect highest
rainfall totals from early Thu onward to extend from along and
just west of the Appalachians into the Northeast. A brief period
of fairly strong winds will be possible over parts of the Great
Lakes/Northeast but details will require better agreement on the
depth/track of the surface low. Behind this system expect periods
of lake effect rain/snow with periodic light precipitation over
other parts of the northern tier in association with a couple
fronts dropping south from Canada. The front trailing from the
late week eastern storm will settle over the Florida Peninsula and
could provide a focus for light rain into early next week.
The Rockies and Plains will remain quite chilly into late week
with a fairly broad area seeing temperatures 10-30F below normal.
These areas should trend much closer to average by Sun-Mon while
locations closer to the West Coast should see highs up to 5-10F
above normal from the weekend onward as the Pacific ridge aloft
extends its influence a little farther eastward. Meanwhile
frontal passage over the East will replace initial warmth (plus
10-20F anomalies for morning lows and moderately warm highs on
Thu) with cooler readings generally 5-15F below normal.
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