Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 AM 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 12Z ECMWF/CMC and 18Z GFS (with
heaviest weight place toward the ECMWF). This blend leaned toward
the slower solutions (which were well supported by ensemble member
clustering) with the shortwave/surface low crossing the Ohio
Valley/Northeast on Sun. Additional northern stream shortwaves,
one crossing the north central U.S., and another diving into the
Pacific Northwest Sun-Mon were handled similarly by these
preferred solutions. Farther south, a cutoff southern stream upper
low approaching southern California late Sun-Mon continued to show
some timing differences, and the slower ECMWF was preferred over
the somewhat faster GFS timing (causing the GFS solution be
further deemphasized by Mon).
During days 6-7 (Wed), timing differences with the southern stream
feature continue to grow as the feature moves into the central
U.S. by Wed, with the GFS at least 18 hours faster than the
ECMWF/CMC. Farther north, 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.
Ensemble means showed general agreement on this idea despite
increasing spread. Thus, the forecast by Wed-Thu placed increasing
weight on ECENS/GEFS ensemble means, with some continued use of
the deterministic ECMWF through the end of the period.
...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.
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. Warming temperatures are expected across the
Southeast and southern Mid-Atlantic by early next week as the
subtropical ridge builds. Highs across these areas may reach 10 to
15 deg above average Tue-Thu.
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