Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Tue May 07 2019
Valid 12Z Fri May 10 2019 - 12Z Tue May 14 2019
...Heavy rainfall threat for eastern/southeastern Texas and the
Lower Mississippi Valley late this week into the weekend...
...Overview...
Expect a stalled front near and northeast from the western Gulf
Coast (anchored by low pressure over eastern Canada as of early
Fri) to provide a focus for periods of heavy rain along and north
of the western-central Gulf Coast region into the weekend. With
continued uncertainty in the details, upper troughing forecast to
amplify into the Plains/Mississippi Valley and then move into the
eastern U.S. may draw some of this moisture northeastward later in
the weekend/early next week while suppressing part of the
front--reducing the heavy rain threat somewhat. Across the West
the upper pattern will begin to look more like a Rex block as a
warm ridge builds over the Northwest while a California upper low
as of Fri eventually drifts along the western U.S.-Mexico border.
This upper low and an impulse tracking to its south and east will
promote multiple days of precipitation over the southern half of
the West. Guidance shows signs of an amplifying trough developing
over the eastern Pacific toward the end of the period next Tue.
...Guidance Evaluation/Predictability Assessment...
Over and near the western U.S. the prevailing multi-day trend is
for a stronger ridge to build into the Northwest and slower
progression of the upper low expected to track near the western
U.S.-Mexico border. In general recent GFS/GEFS runs have been
somewhat ahead of the ECMWF/ECMWF mean with these trends. The 12Z
ECMWF made a favorable adjustment relative to its faster prior
runs so it could be included in the forecast through the period,
while aforementioned trends seem to favor somewhat more GEFS mean
than ECMWF mean for the ensemble input by the latter half of the
period. The 12Z CMC mean looks quite suspicious with a rather
fast moving eastern Pacific trough that reaches the West Coast by
day 7 Tue. Some details remain complex, in particular for an
impulse that likely tracks south of the California upper low
around Fri and may continue into the southern Rockies/Plains--but
with the potential for a portion of the energy to curl back toward
the upper low.
Evolution of upper troughing expected to amplify over the
Plains/Mississippi Valley during the weekend--and the associated
surface reflection--continue to be a significant forecast problem.
Among guidance from the 12-18Z cycles there were essentially two
clusters. The slower/sharper/more separated group of the
ECMWF/UKMET/CMC aloft led to a weaker and more suppressed surface
feature while the more consolidated GFS/GEFS mean/CMC mean had a
stronger/northward surface system. The one thing in common is a
multi-day trend toward the trailing cold front reaching farther
southward. Operational GFS runs were a little suspicious in how
they phased northern Ontario energy into the original shortwave so
they might be too strong. The 18Z FV3 GFS did not show such
phasing and the new 00Z GFS has trended away from that as well.
Beyond that the details appear too fine in scale to resolve
confidently at the extended time frame, thus favoring a blended
approach. Behind this system additional shortwave energy (again
with low confidence for specifics) will complicate the forecast
within the overall mean trough.
Specifics of the amplifying trough will have a significant
influence on the duration of the potential heavy rain event over
the South and how much moisture may stream farther northeastward.
The ECMWF cluster described above would lead to more
moisture/frontal waviness reaching northeastward and a shorter
duration of rainfall over the Gulf Coast states. Again preferred
an intermediate approach given lack of confidence in northern
stream trough specifics. Note that the impulse tracking south of
the California upper low on Fri may also play a role in promoting
a frontal wave over the East. In spite of the differences over
the South and East during the weekend, there is actually some
convergence of solutions toward a wave off the Mid-Atlantic/New
England coast early next week. Trends in the 00Z GFS and other
incoming 00Z models suggest the 12-18Z GFS solutions are less
likely.
...Weather Highlights/Threats...
The best confidence in heavy rainfall potential over
eastern/southeastern Texas through the Lower Mississippi Valley
will be in the Fri-Sat time frame. Then latest trends show an
increasing possibility that one or more frontal waves lifting
through the eastern states may pull areas of significant rainfall
into the Tennessee Valley and Mid-Atlantic while reducing
precipitation over parts of the Gulf Coast region. However the
front extending into the western Gulf and northern Mexico will
still be close enough to maintain potential for rainfall of
varying intensity through the first half of next week. Elsewhere
over the eastern half of the country, the storm over eastern
Canada early Fri will bring a brief period of enhanced rainfall to
the Northeast while the system dropping into the
Plains/Midwest/Great Lakes during the weekend should be
accompanied by mostly light-moderate precipitation.
Over the West, continue to expect terrain-enhanced precipitation
from portions of California into the southern half of the Rockies.
Highest 5-day totals should be over the Rockies. Isolated
highest elevations of the Rockies could see a little snow but with
much lower potential than will be the case before Fri. Western
areas should gradually trend drier but overall expect a longer
duration of precipitation given slower trends for the upper low
tracking near the U.S.-Mexico border. Eventually the upper low
should increase activity over the southern High Plains.
Expect very warm temperatures over the Northwest for multiple days
as the upper ridge builds over the region. There should be decent
coverage of plus 10-20F anomalies with locally higher departures
from normal. Daily records for highs/warm lows will be possible
and some areas may approach 90F. Meanwhile the forecast pattern
will keep the southern Plains through Southwest well below normal
for highs. The southern High Plains should see the most extreme
anomalies of minus 15-25F for highs on Fri. Areas from the
Midwest to Northeast will be moderately cool during the period,
5-10F below normal and locally a bit cooler.
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