Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Sun Oct 13 2019
Valid 12Z Wed Oct 16 2019 - 12Z Sun Oct 20 2019
...Pattern Overview and Guidance/Uncertainty Assessment...
Today's guidance continues to show a relatively amplified but
progressive pattern across the lower 48 during the extended
period. A deepening upper trough should develop an embedded closed
low as it tracks through the Great Lakes/Northeast Wed-Thu, while
surface low pressure and frontal system cross the eastern U.S.
ahead of the trough. As the initial surface low passes through the
Great Lakes on Wed expect a new surface low to form across the
Mid-Atlantic region. This southern low should quickly intensify as
it moves northeastward along the New England coast Wed night-Thu.
Farther west an intense Pacific upper-level jet and train of
shortwave energy will reach the Pacific Northwest starting Wed
night. This should quickly lower heights across the Northwest and
eventually the northern Rockies as the systems move inland. By Fri
expect the first significant shortwave in this series to exit from
the Rockies into the central U.S. and push a leading cold front
into the Plains. A decent majority of model/ensemble guidance
suggests that a more significant trough will amplify into the Four
Corners region or Rockies next weekend, moving into the Plains
around or beyond the end of the forecast period.
The most significant uncertainty in terms of guidance spread and
associated sensible weather revolves around the upper trough
expected to amplify into the western/west-central states next
weekend. Over recent days nearly all operational models have been
varying between a broader/open trough (such as the 00Z GFS, and to
some degree the 00Z ECMWF that waits until reaching the northern
Plains to close off an upper low) and slower/deeper/closed
depiction (most prominently the 00Z CMC; 06Z GFS falls between the
00Z ECMWF and CMC) with major implications for precipitation
coverage and intensity over portions of the Rockies and Plains.
For a specific deterministic forecast the ensemble means have
provided the most consistent overall forecast of a fairly
amplified trough (with the 06Z GEFS trending a little
slower/sharper from the 00Z run). Thus forecast preferences lean
more toward the means by days 6-7 Sat-Sun. The 06Z GFS and 12Z/12
ECMWF runs provide the most representative/intermediate
operational solutions for a possible embedded closed low by late
Sun.
For the most part guidance shows better agreement for days 3-5
Wed-Fri, favoring a model average in this time frame. This blend
tends to give somewhat lower weight to GFS runs that in varying
ways differ somewhat from the majority cluster for the deep storm
that reaches near New England by Thu. The GFS has been trending
gradually slower with the shortwave energy and leading surface
front crossing the Northwest and central U.S., though it may still
be a tad fast. Meanwhile most guidance shows an area of modestly
lower surface pressures over the western/southwestern Gulf of
Mexico. Sporadic operational runs have tried to bring a
better-defined surface low toward the Gulf Coast but with general
trends to back away from that scenario in later runs. Currently
the 06Z GFS looks particularly questionable with its low reaching
Louisiana by Sat, so the early-middle part of the forecast uses
the 00Z run for GFS input.
...Weather Highlights/Threats...
Expect widespread and at times locally heavy precipitation over
the Pacific Northwest from midweek through next weekend as
multiple shortwaves and surface fronts affect the region.
Snow-levels should fall as lower heights aloft press inland. The
heaviest precipitation is likely to fall across the coastal ranges
as well as the Cascades, with multi-inch liquid equivalents
possible. Less extreme precipitation should reach into extreme
northwest California and favored terrain of the northern Rockies.
The upper trough expected to amplify over the West next weekend
will spread precip farther east/southeast into the Rockies.
However confidence in exact coverage and intensity is low due to
considerable uncertainty over how the upper trough will evolve.
The central/east-central U.S. will likely see increasing coverage
of rainfall by next weekend ahead of the amplifying western upper
trough. High pressure moving from the eastern U.S. into the
western Atlantic will promote a strengthening low level flow of
moisture from the Gulf of Mexico and a front over the Gulf
returning northward may provide a focus for heavier rain along
parts of the Gulf Coast.
The deep storm affecting the Northeast during the latter half of
the week should bring heaviest precipitation to New England along
with a period of fairly strong winds. Nearly all precip should be
in the form of rain but it could be possible for highest
elevations to see a little snow as the upper low passes through.
Much of the central and eastern U.S. will see relatively cool
conditions Wed into Thu with highs for many areas 5 to 15 deg F
below average. These temperatures should warm by Fri and into next
weekend. The warming trend should be more pronounced for min
temperatures next weekend with the stronger flow of Gulf moisture.
Meanwhile initially above average temperatures across much of the
West (highs 5 to 10 deg F above normal) will be replaced by much
cooler conditions as upper troughing moves inland. Highs of 5 to
10 deg F below normal are likely over the Northwest by Thu. By
next weekend colder conditions should overspread much of the
western U.S. as the main upper trough axis presses inland and
amplifies, leading to highs 10 to 15 deg F below normal over some
areas.
Rausch/Ryan
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