Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
202 AM EDT Sat May 25 2019
Valid 12Z Tue May 28 2019 - 12Z Sat Jun 01 2019
...Record/Dangerous Heat expected across the Southeast with heavy
rain possible for portions of the central U.S...
...Overview and Guidance Evaluation/Preferences...
Models/ensembles showed broad agreement at large scales through
the medium range. A broad upper-level low is expected to persist
in the vicinity of Hudson Bay, with persistent positive height
anomalies across the Southeast U.S. associated with the
subtropical ridge. Farther west an upper ridge axis is expected to
persist from the Pacific Northwest into western Canada, with
Pacific shortwave energy undercutting the ridge continuing to
reinforce a mean trough across the Southwest/Great Basin before
progressing east into the central U.S. This pattern will keep the
focus for frequent areas of low pressure and precipitation across
portions of the Plains and the Mississippi Valley - unwelcome news
given the extremely wet conditions and flooding concerns across
those areas in recent weeks.
Agreement among the guidance with respect to some of the forecast
details has improved somewhat over the past day, with at least a
couple waves of low pressure expected to traverse a frontal
boundary across the central and northeastern U.S. days 3-5
(Tue-Thu), with a relatively strong secondary front entering the
Midwest/Great Lakes by Thu. A blend of deterministic guidance
resolved many of the remaining differences with respecting to
track/timing of these features, and represented a good consensus.
Thus, the forecast for days 3-5 was based on a blend of the 12Z
ECMWF/UKMET and 18Z GFS. By Fri-Sat (days 6-7) a vigorous
shortwave should dig into the Midwest/Great Lakes with a leading
cold front spreading across the Northern Plains. The arrival of
additional southern stream shortwave energy should once again
spark the development of a surface wave of low pressure across the
Central Plains by Fri night-Sat. Models/ensembles generally agree
on this overall scenario, but with some significant detail
differences given the time range. Thus, use of deterministic
guidance was tapered in favor of majority ECENS/GEFS ensemble
means in the forecast during days 6-7.
...Weather Highlights and Hazards...
High temperatures should be 10-20 deg F above average across much
of the Southeast Tue-Thu with readings likely nearing or
surpassing 100 deg F for many areas along/below the fall line.
These temperatures will combine with moderate humidity levels to
produce dangerous heat index values that could at least briefly
exceed 105 deg over some areas. There will be multiple days with
potential for record highs/warm lows. In less extreme fashion some
of this heat should spread northward into portions of the
Mid-Atlantic through midweek. A front will provide a cooling trend
later in the week. Meanwhile the upper trough initially ejecting
from the West will lead to cool conditions with high temperatures
10-20 deg F below average from California through the Great
Basin/north-central High Plains Tue. Temperatures should gradually
trend closer to average mid-late week. At the same time upper
ridging will be close enough to the Northwest to provide above
normal temperatures for most of next week.
Heavy rainfall will be a threat across portions of the central
U.S. during the next week. The low pressure system crossing the
central U.S. early-mid week could produce multiple rounds of
potentially heavy rainfall and severe thunderstorms across
portions of the Plains and the Midwest, with multi-inch rainfall
totals possible. Some enhanced rainfall could extend into the
Great Lakes/Northeast with a leading frontal wave, and then later
in the week as the front trailing from the main Plains system
reaches the area. Later in the week the trailing end of the front
should linger across the Southern Plains and Lower/Mid-Mississippi
Valley where it could continue to focus areas of showers and
storms with at least locally heavy rainfall. The next shortwave
and developing low pressure system across the central U.S. could
once again enhance the rainfall potential for portions of the
Southern/Central Plains by Fri-Sat. Across the West the
most-focused precipitation should be associated with the upper low
tracking out of the across the Rockies Tue while residual
troughing aloft could favor periods of diurnally favored activity
through the rest of the week.
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