Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
246 AM EDT Sun May 26 2019
Valid 12Z Wed May 29 2019 - 12Z Sun Jun 02 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, with some significant differences remaining with
respect to the details. A broad upper-level low is expected to
persist in the vicinity of Hudson Bay, with persistent positive
height anomalies centered across the northeastern Gulf of Mexico
associated with the subtropical ridge. Farther west an upper-level
ridge axis is expected to persist from the Pacific Northwest into
western Canada, with Pacific shortwave energy undercutting ridge
reinforcing a mean trough across the Southwest/Great Basin before
progressing east into the central U.S. This pattern will keep the
focus for the development of areas of low pressure and resultant
precipitation across portions of the Plains and the Mississippi
Valley.
A blend of the 12Z ECMWF/UKMET/18Z FV3 was used as a basis for the
forecast during days 3-5 (Wed-Fri). Differences emerged fairly
early on with northern stream shortwave energy rotating around the
broader Hudson Bay upper low, and its interaction with southern
stream energy crossing the Midwest/Great Lakes Wed-Thu. The GFS
and CMC were significantly slower with northern stream energy,
with a delayed interaction with the southern stream feature
resulting in a significantly slower timing for the surface cold
front across the Great Lakes/Northeast through Fri - and those
solutions were thus excluded from the forecast due to a lack of
sufficient ensemble support.
Additional northern stream shortwave energy is forecast to reach
the Northern Plains on Fri, with the development of a wave of low
pressure expected along a secondary frontal boundary as it
continues to move east during days 6-7 (Sat-Sun). Timing and
amplitude differences with this wave translate to fairly
significant differences with respect to the track/intensity of any
surface wave(s) of low pressure along the front. Finally,
shortwave energy riding an active upper jet across the North
Pacific should push a cold front into the Pacific Northwest by
next Sun. Models showed somewhat better consensus on this feature
relative to features farther east, with ensembles and
deterministic solutions all showing similar timing for the upper
wave, and some amplitude differences. The preference at this time
was to stick fairly close to the ensemble means during this time
period given the spread. The 12Z ECMWF and 18Z FV3 solutions
continued to appear somewhat well-centered relative to the means
through this time period. Thus, the forecast during days 6-7 was
based on a blend of majority ECENS/GEFS ensemble means, with
minority components of the ECMWF/FV3 continued through the end of
the forecast period.
...Weather Highlights and Hazards...
High temperatures should be 10-20 deg F above average across much
of the Southeast Wed-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. 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
10-15 deg F below average from the Great Basin to the High Plains
on Wed. Temperatures should gradually trend closer to average by
late week. At the same time upper ridging will be close enough to
the Northwest to provide above average temperatures (generally
plus 5-15 deg F anomalies) for most of the week.
Heavy rainfall will be a threat across portions of the
Southern/Central Plains and perhaps the Mid-Mississippi Valley
during the upcoming week. A trailing cold front from the system
crossing the Great Lakes Wed-Thu could produce widespread showers
and thunderstorms with potentially heavy rainfall and severe
weather across portions of the Southern Plains, with localized
heavier rainfall amounts possible northeast along the front into
the Mid-Mississippi Valley. The next shortwave along with the
northward progression of a weakening warm front could once again
enhance the rainfall potential for portions of the
Southern/Central Plains Fri-Sat. Across the West, largely terrain
focused precipitation is expected from the Great Basin to the
central/northern Rockies through much of the week with the passage
of several upper-level shortwaves during the period
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