Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1150 AM EDT Sat May 04 2019
Valid 12Z Tue May 07 2019 - 12Z Sat May 11 2019
...Overview...
Various details remain uncertain but guidance is continuing to
suggest that a strong eastern Pacific ridge will promote western
U.S. troughing, while a broad/flat ridge will likely be oriented
near or just offshore the East Coast. An ejecting Southwest U.S.
upper low Tue onward and associated surface wave/frontal system,
followed by slow movement/possible stalling of the trailing front
as it becomes aligned with southwesterly flow aloft, will produce
a heavy rainfall threat over parts of the Plains and Mississippi
Valley. Some affected areas will likely already have wet ground
thus elevating flash flood concern. A less extreme portion of
this moisture should extend into the East, while the ejecting
Southwest upper low and subsequent larger scale troughing should
generate terrain enhanced precipitation over parts of the West.
...Guidance Evaluation/Predictability Assessment...
In the Tue-Wed time frame there had been considerable uncertainty
over details across the northeastern quadrant of the Lower 48 into
Canada. Starting with yesterday's 06Z GFS there has been better
agreement in principle on the more suppressed scenario which the
majority cluster had shown 24 hours ago. There has still been
somewhat of a southward trend with the northeastern front dropping
into the Mid-Atlantic.
As the ejecting Southwest U.S. upper low reaches the Rockies and
beyond, guidance displays typical divergence for timing--due in
part to differences that develop within the amplifying western
trough. Overall prefer a broader consensus timing which was
slower than the 00Z Canadian and more defined than the 00Z UKMET.
Over the past day guidance has trended stronger for the surface
wave reaching the Midwest by early Thu, reflecting better
definition of the upper system. Also worth noting, recent GFS
runs remained stronger than other models with a leading Ohio
Valley to eastern U.S. wave at the surface and aloft Wed-Thu
unlike the other models (including the 06Z FV3-GFS), so that wave
was deemed overdone.
The rest of the forecast involves more broadly dispersed
models/ensemble members for specifics. 00Z GFS runs was among the
more western or southwestern solutions for the upper low that may
close off within the western U.S. trough aloft but the 00Z ECMWF
and its ensemble mean shifted westward from its earlier run.
Preferred the 06Z GFS with the 00Z ECMWF along with continuity to
mitigate future shifts in the evolution of the western trough,
given the recent ensemble changes.
Separate troughing (with possible embedded low) over southern
Canada and extending into the northern tier of the Lower 48 will
provide added forecast challenges, especially during the latter
half of the period. Thus far guidance has not been very agreeable
or consistent with the associated wave(s)/front(s) reaching the
northern U.S.--maintaining low confidence in any specific
solution. Latest 00Z ensembles indicated a much more robust system
entering the High Plains late Fri into Sat in addition to a
stronger and perhaps more northern sfc low into the eastern Great
Lakes Friday. Formed a best consensus among the 06Z GFS/00Z ECMWF
and the ensemble means to take that front/wave off the Northeast
coast early next Saturday, but with low confidence in anything
other than broad details.
...Weather Highlights/Threats...
Continue to expect a threat of heavy rainfall over the
central/southern Plains and Mississippi Valley through much of the
period (Tue-Fri). Less extreme but still significant rainfall may
extend into the the Ohio/Tennessee Valleys and
Northeast/Mid-Atlantic but with lesser confidence on specifics.
The central/north-central Plains activity will likely take place
during the first half of the period. A longer duration of heavy
rain/thunderstorms will be possible farther south--in advance of
the system emerging from the West into the Plains and then late
week into the weekend as persistent low level flow from the Gulf
interacts with a front that may stall for a time over the Lower
Mississippi Valley/western Gulf Coast. The wave reaching the East
late in the week will provide the primary focus for enhanced
rainfall over that part of the country.
Across the West, anticipate varying intensity of precipitation
from California through the Great Basin/northern Arizona and into
most of the Rockies as the upper low tracks through southern areas
early in the week and the upper trough/possible embedded upper low
develop over the West later in the week. Some of the precipitation
should be in the form of snow over high elevations of the
northern-central Rockies. Depending on exactly how features evolve
and time of day, some snow could extend into the north-central
High Plains. Highest precip totals are most likely over favored
terrain within the Rockies from Wyoming to northern New Mexico.
Some locations farther west may see significant amounts as well
including southern Utah.
Chilly air pushing southward over the Plains may bring highs down
to 15-25F below normal over north-central/central High Plains on
Wed-Thu, with a broader area of below normal readings over the
central U.S./Rockies and Southwest during most of the period. The
system tracking out of the Southwest will bring warmth into the
East later in the week with low temperatures in particular 10F or
greater above normal but high temperatures about 5-10F above
normal. The Pacific Northwest should see multiple days with highs
10-20F above normal and stand the best chance of tying/breaking
record highs.
Fracasso/Rausch
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