Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1155 AM EDT Sun Jun 03 2018
Valid 12Z Wed Jun 06 2018 - 12Z Sun Jun 10 2018
...Heat wave from the Desert Southwest to the south-central U.S...
...Pattern overview...
A persistent mid/upper-level ridge will maintain control of the
pattern with a general position over the south-central U.S. With
an area of lower heights remaining over the West Coast, many such
impulses are forecast to act as "ridge rollers" as they track
downstream toward the Great Lakes and into the Ohio Valley. These
features should eventually adjoin the mean upper trough across the
northeastern U.S. The overall set up will likely change very
little this week.
...Model guidance/predictability evaluation/preferences...
An initial closed low over southern New England will be
accompanied by a modest area of low pressure off the Delmarva
coast. This well clustered system is forecast to eject east of
Nova Scotia by Thursday morning. Meanwhile the focus shifts back
toward a notable perturbation in the flow straddling the southern
border of Manitoba mid-week. The solutions do agree better with
the poleward expansion of the sprawling upper ridge across the
south-central states. Where they disagree is with mesoscale
details of evolving impulses off the upstream higher terrain. At
this forecast juncture, it is too early to resolve such spatial
scales at this time. It is safe to say multiple convective
episodes will be likely north and east of this ridge with activity
congregating from the Northern Plains downstream into the Great
Lakes/Ohio Valley regions.
Across the eastern Pacific, a unseasonably strong upper cyclone
with 500-mb height departures on the order of 2 standard
deviations below climatology will be a key player in the forecast.
While the strongest forcing/height falls should swing through
British Columbia and Alberta, there are some operational models
and ensembles favoring a marked lowering of heights over the
Pacific Northwest. Recent GFS runs in particular depict such a
solution, albeit with poor run-to-run model continuity and outlier
status without almost any ensemble support.
Overall, good model agreement into Day 5/Fri allowed for a WPC
multi-operational model consensus utilizing the 06 UTC GFS and the
00 UTC ECMWF/UKMET/Canadian. Quickly transitioned to a reasonably
clustered 06 UTC GEFS mean and 00 UTC NAEFS/ECMWF ensemble means
blend days 6/7 amid growing forecast spread/uncertainty.
...Weather highlights and hazards...
It remains the case that the major weather story will be the
abundant heat filling out the Desert Southwest eastward into the
Southern Plains. This is in response to the persistent upper ridge
holding strong over the region. Forecast highs will be in the
triple digits over many of these regions, particularly the
California, Arizona, and Nevada deserts as well as the state of
Texas. Temperature anomalies should maximize over the Southern
Plains with departures around 10 to 15 degrees above climatology.
On the contrary, an upper trough departing the northeastern U.S.
will keep temperatures on the cooler side for Wednesday/Thursday
as readings hold into the 60s over much of New England as lows
drop into the 40s.
It should meanwhile prove to be an unsettled start to the period
with cool cyclonic flow maintaining showers over the northeastern
U.S. on Wednesday while thunderstorms are likely across the
southeastern U.S. along the advancing cold front. Farther
upstream, a series of convective complexes will evolve north of
the upper ridge with hefty rainfall amounts likely from the
Northern High Plains eastward into the Middle Mississippi Valley
and eventually toward the Ohio Valley. While excessive rainfall
concerns are somewhat unknown, some convectively driven hazards
are likely with any such complex. Across the Pacific Northwest, it
could prove to be quite wet if the trough amplifies as much as
suggested. While extreme, recent outlier GFS runs show impressive
coverage of QPF for early/mid June, but such potent height falls
looks way overdone.
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be found at:
http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
Schichtel