Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1158 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...
Guidance continues to agree well for the large scale evolution
through next Sat but embedded detail questions remain. A split
flow pattern will settle over western North America during the
latter half of next week. Within this pattern a northeastern
Pacific ridge will build into western Canada to replace an initial
ridge that collapses southeastward, while eastern Pacific
shortwave energy will drop into a persistent Great Basin/Southwest
upper trough. The trough will not be as deep as the one crossing
the area in the short range time frame. Farther east expect the
mean flow to flatten with time as the northern Plains through
Northeast come under greater influence from cyclonic flow to the
south of an upper low near Hudson Bay, and East Coast ridging
anchored by an eastern Gulf/Florida upper high through midweek
loses amplitude.
Over central/eastern North America days 3-5 Tue-Thu the primary
forecast uncertainties continue from prior cycles. There is still
some variability with the amplitude of cyclonic flow reaching the
northern tier states, affecting the associated front that drops
south from Canada Tue-Thu. However the past couple cycles or so
have become a little more stable for timing of system tracking
through the Upper Midwest. The 00Z CMC is in the minority with a
faster solution. By Thu the latest GFS runs are somewhat faster
than the majority scenario (including the 06Z GEFS mean) with weak
trailing energy--ultimately affecting cold front progression over
the East. Meanwhile confidence remains low for exact wave details
along the leading front that extends through the Northeast during
the first half of the period. The 00Z UKMET was in the minority
at some valid times over this region. Based on these
considerations the updated forecast started with the 00Z
ECMWF/ECMWF mean and 06Z GFS/FV3 GFS/GEFS mean with about 2/3
weight toward the operational runs. The result tilted the blend
away from the more questionable GFS details over the East by
Wed-Thu.
By days 6-7 Fri-Sat there was reasonable overall agreement among
the models and means. Thus the only adjustment from the earlier
portion of the forecast was to increase ensemble weight to 40-60
percent while maintaining the same components. Note that the 00Z
GFS appeared overdone with height falls nearing the West Coast by
day 7 Sat. Meanwhile consensus shows another front pushing into
the northern tier states from Canada by Fri-Sat and weakening of
the front that reaches the East/South. The southern Plains part
of the front may lift back as a warm front late in the week.
...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 (generally plus 5-15 deg F anomalies) 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. Tue-Wed and trailing front 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 extending from the main Plains
system reaches the area. Later in the week the trailing end of the
front should linger for a time across the Southern Plains and
Lower 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 crossing the northern
Plains as well as northward return of the front settling over
Texas could once again enhance the rainfall potential for portions
of the central U.S. by Fri-Sat. Across the West, the upper low
reaching the central Rockies by early Tue will generate focused
precipitation (possibly some snow at highest elevations) in its
vicinity very early in the period. Persistent troughing aloft
could favor periods of diurnally favored activity through the rest
of the week into the weekend.
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