Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
325 PM EDT Thu Apr 25 2019
Valid 12Z Sun Apr 28 2019 - 12Z Thu May 02 2019
...Overview and Guidance Preferences...
An expansive Rex Block across the north/central Pacific, extending
from Hawaii to Alaska, will support a downstream mean upper trough
centered across the northern Rockies through the extended forecast
period. A persistent upper low centered west of Hudson Bay, and
slowly retrograding through the period, will support southward
transport of chilly polar air into the north central U.S. Farther
east, the subtropical ridge is expected to gradually exert its
influence northward into the Southeast and southern Mid-Atlantic
early next week.
Preferred a multi-model blend approach initially during days 3-5
(Sun-Tue), incorporating the 00Z ECMWF/GFS/UKMET/Canadian. This
blend leaned toward the slower solutions with the southern stream
upper low approaching southern California late Sun-Mon that
continued to show some timing differences. Was not as slow as the
ECMWF/Canadian but not as fast as the GFS which ended up near the
00Z UKMET. As it moved through AZ early Tuesday, removed the GFS
from the blend as both 00Z/06Z runs became quicker than the rest.
In turn, favored the slower 06Z FV3-GFS which clustered near the
UKMET. Exiting low through the Northeast and incoming weakening
upper low over Montana were handled well by the consensus blend to
start.
During days 6-7 (Wed-Thu) model consensus has improved a bit that
the arrival of additional shortwave energy into the Pacific
Northwest early-mid next week should serve to further amplify a
broad upper trough axis centered over the northern Great
Basin/northern Rockies, with a consolidated area of shortwave
energy perhaps lifting out into the Northern Plains by Thu. GEFS
and ECMWF ensemble means showed general agreement on this idea
despite increasing spread, but the shape of the western trough and
upstream ridge/trough couplet out of the northeastern Pacific into
the Pac NW differed between those means, as well as the northern
extent of the upper high along the Southeast coast. Preferred to
side more with the ECMWF ensemble mean by next Thu as it had a
more coherent surface low lifting toward the Great Lakes which was
the broad idea of the deterministic models (the GEFS mean had too
much high pressure). Kept the 00Z ECMWF in for some added details
as it fit this idea best.
...Weather Highlights/Threats...
Showers and perhaps a few thunderstorms are expected to accompany
the low pressure system moving from the Ohio Valley/lower Great
Lakes to the Northeast on Sun. After that, the focus shifts to the
central U.S., where a persistent stalled frontal boundary along
with a few passing upper shortwaves will focus multiple rounds of
showers and thunderstorms, with the potential for areas of heavy
rainfall. The strongest signal in the guidance for heavy rain was
seen from portions of the Southern/Central Plains to the
Mid-Mississippi Valley from Mon through the middle of next week,
with QPF guidance suggesting the potential for multi-inch rainfall
totals over the course of a few days. On the northern fringe of
this area of precipitation in the central U.S. sufficient cold air
may be in place to support some snow on Sun across portions of the
Northern Plains. A second round of potential snow or rain/snow mix
will be possible across portions of the Northern High plains
Tue-Wed. Farther west, lowering heights and the passage of
multiple shortwaves will generate rain and mountain snow for
portions of the Great Basin and central/northern Rockies through
the period. Severe weather will be an almost daily possibility in
the warm sector of the Southern Plains from Mon-Thu, but the focus
for now is around Tuesday where the Storm Prediction Center has
outlined a 15% area across much of Oklahoma into north central
Texas.
Below average temperatures are expected from Sun into next week
from the northern Rockies to the Great Lakes. The greatest
anomalies are expected Sun-Wed from the northern Rockies to the
Northern High Plains where high temperatures may be 10-20 deg F
below average (mainly in the 40s). Warming temperatures are
expected across the Southeast and southern Mid-Atlantic by early
next week as the subtropical ridge builds northward. Highs across
these areas may reach 10 to 15 deg above average Tue-Thu.
Temperature spread in the ensembles remained quite large east of
the Appalachians and along the frontal boundary to the west as the
difference between cool northeasterly flow off the cold Atlantic
waters vs a warm downsloping southwesterly flow translates into
50s vs 80s for high temperatures in the mid-Atlantic. Higher
confidence existed at least a couple hundred miles south or north
of the forecast frontal position for warmer or cooler temperatures
compared to average, respectively.
Fracasso/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
Hazards:
- Heavy rain across portions of the Central/Southern Plains, the
Middle/Lower Mississippi Valley, the Great Lakes, and the Ohio
Valley, Sun-Thu, Apr 28-May 2.
- Heavy snow across portions of the Northern Rockies, Sun, Apr 28.
- Heavy snow across portions of the Northern/Central Rockies
Mon-Wed, Apr 29-May 1.
- Heavy snow across portions of the Northern Plains (Bighorn
Mountains), Tue-Wed, Apr 30-May 1.
- Severe weather across portions of the Middle Mississippi Valley,
the Lower Mississippi Valley, and the Southern Plains, Tue, Apr 30.
- Flooding possible across portions of the Great Lakes and the
Southern Plains.
- Flooding occurring or imminent across portions of the
Mississippi Valley, the Northern and Southern Plains, the
Tennessee Valley, the Great Lakes, the Northern Rockies, the
Northern Great Basin, the Ohio Valley, and the Northeast.
- Flooding likely across portions of the Northern Great Basin.
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