Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
246 AM EDT Sat Jun 23 2018
Valid 12Z Tue Jun 26 2018 - 12Z Sat Jun 30 2018
...Overview and guidance/predictability assessment...
Over the course of the period the majority of guidance is fairly
consistent and agreeable in showing the establishment of a western
U.S. trough while a mean ridge becomes more prominent over the
eastern half of the country. After passage of a northern
Plains/upper MS Valley through New England shortwave during the
first half of the period, the expanding eastern ridge should
deflect the westerlies well northward, into the Great Lakes/New
England. At that time this area would also represent the
southeast periphery of a broad cyclonic circulation covering the
eastern half of Canada.
The good continuity of the ensemble means thus far, and
teleconnection support for the western trough (and to some degree
flow downstream) relative to the building eastern Pacific ridge,
suggest the pattern has above average predictability. However
some individual model runs and ensemble members maintain the
potential for a somewhat different outcome. The 12Z GFS seemed to
provide one of the less likely solutions, with a faster
late-period ejection of western trough energy into the Plains
versus all other 12Z GEFS/ECMWF/CMC ensemble members at some
locations. 18Z-00Z GFS runs have adjusted back to the consensus
trough axis but with an embedded upper low/trough core somewhat
deeper and/or south of the majority cluster by days 6-7 Fri-Sat.
On the other hand CMC runs have tended to be somewhat flatter with
the overall eastern Pacific/western U.S. pattern. The 00Z version
compares better to other guidance than the prior run though.
Uncertainty over the precise character of the western trough leads
to some spread for downstream flow, with ensemble spread for
576-582dm height contours covering much of southeastern Canada and
northeastern U.S. by late in the period. Questionable western
trough progression in the 12Z GFS led to corresponding
questionably high heights over eastern North America late in the
forecast.
Another forecast uncertainty during the period will involve the
ultimate evolution of the closed low initially over western Canada
and flow to its east/southeast. Solutions show meaningful
divergence for details from about day 5 Thu onward. This will
affect the handling of surface front(s) affecting the northern
states. Ahead of this feature there is better than average
agreement for the northern Plains/upper MS Valley system that
opens up as it progresses east-northeastward.
The updated forecast blend yielded only the typical run to run
detail variability, with good continuity for significant features.
The 18Z GFS and 12Z ECMWF/UKMET/CMC provided input for about the
first half of the period. Then the forecast incorporated 50-60
percent total of the 18Z GEFS/12Z ECMWF means by days 6-7 Fri-Sat
given their recent stability and to downplay lower confidence
operational model specifics. Poor comparison to consensus led to
removal of the CMC from the blend after day 5 Thu.
...Weather threats/highlights...
The system tracking east-northeast from the northern Plains/upper
MS Valley Tue onward will likely produce one or more episodes of
locally heavy convection. Best potential for areas of enhanced
rainfall should exist from the upper-mid MS Valley through the
Great Lakes and Ohio Valley. Some pockets of moderate to heavy
rainfall may extend into the Northeast but should generally be
more scattered as the upper system weakens. The trailing front
progressing across the northern tier states will stall over the
Plains with at least a portion likely to return north as a warm
front by Fri-Sat. The combination of this low level focus and
increasing tendency for shortwave impulses to eject from the
evolving western trough aloft may promote periods of
convection/rainfall--some potentially heavy--from the northern
Rockies eastward by late week/weekend. The western trough should
also bring unsettled weather to other parts of the northern half
of the West though with lighter amounts. Meanwhile expect periods
of convection over the Southeast/Florida during the period with
some heavy activity possible. A gradually weakening front over
the region will provide some added low level focus during the
first half of the period.
The forecast is still on track for hot/humid conditions to become
established over an increasingly large portion of the
central/eastern states. Locations from the central Plains to Mid
Atlantic/Northeast will have the best potential for seeing one or
more days with min and/or max readings reaching as high as 10-15F
above normal. Adding in the effects of humidity, highest heat
index values should expand from the South early in the period to a
large area centered over/near the mid MS Valley as well as
southern Mid Atlantic/Southeast. The Great Basin/Four Corners
region will see warm temperatures but with some gradual cooling
later in the period with the arrival of the western trough aloft.
This feature should bring coolest anomalies (5-15F below normal
for highs) into the northern parts of the West by next Sat.
Rausch
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities are 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