Scientific Forecaster Discussion
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Area forecast discussion...update
National Weather Service Riverton Wyoming
1133 am MDT Fri Jun 23 2017
Short term...Friday through Sunday night
A shortwave moving through the area at this time is producing a few,
emphasis on a few, showers across northern and eastern areas. The
atmosphere is still rather dry and the rain is probably nothing
more than sprinkles or very light rain. The GFS is depicting this
very well so it will be followed this morning. These showers
should clear the area by morning. The balance of the day will be
dry across the entire area. It will also be rather cool for this
time of year with with some areas seeing highs in the 60s. We did
raise high temperatures a couple of degrees though since there
will be plenty of sunshine with the very high late June sun angle.
A couple of models also continue the Bighorn Rain Forest effect
in the afternoon. This again does not look right.
Another piece of energy will swing through the area on Saturday.
This one may have a bit more moisture to work with though. So for
now, we have kept the probability of precipitation across the northern mountains, but kept
them in the slight category. Most of southern two thirds of the
County Warning Area should remain dry. Temperatures should be fairly
similar to friday's high temperatures. A few areas may also see a
gusty breeze, but nothing extreme is expected. Ridging is then
expected to build across the area on Sunday and bring mainly dry
weather along with temperatures returning near seasonal levels.
Long term...Monday through Friday
Overview...much warmer conditions will prevail Monday and Tuesday
with chances of thunderstorms across the north, breezy to windy
conditions and elevated fire danger across the south. A cooling
trend will follow Wednesday through Friday, as a northwest flow
takes shape. Chances of afternoon/evening thunderstorms will
continue across the north and east.
Discussion...synoptic pattern Monday morning features upper ridge
running along roughly 110w, with surface high over the plains, heat
low over the Great Basin and desert SW. Upper ridge is forecast to
translate the area Monday-Monday night with a couple of Pacific
shortwaves moving through the ridge. Lee side surface trough is
expected to develop through the day Monday with west winds
increasing across the southern wind corridor. Shortwave(s) moving
east across the area will likely be accompanied by a few high-based
showers and thunderstorms that could produce some strong wind gusts.
A more potent shortwave is expected to move across northern Wyoming
on Tuesday, with a Pacific cold frontal passage during the day. The
front will bring gusty winds, with any thunderstorms bringing even a
greater potential for strong wind gusts. Temperatures are not
expected to be noticeably cooler until Wednesday.
Upper low currently over the western Gulf of Alaska is expected to
translate the northern rockies mid-week. The European model (ecmwf) remains faster
and further north than the GFS with its track, but spread has
decreased over the last couple of runs. This system will flatten
the ridge and set up a more zonal flow across the area Wednesday,
transitioning to a more unsettled northwest flow by the end of the week as
ridge rebuilds along the West Coast.
VFR conditions to prevail at all terminals through 12z/Sat
morning. Sky clear to localized scattered cigs 070 ft east of The Divide
and 090 feet west throughout the forecast.
Any stray showers up north should end by day break with mainly dry
conditions expected through Friday night. Temperatures will remain
about 10 degrees below seasonal averages. A bit of a breeze will
continue in some locations but wind should be lighter than the
previous couple of days. Relative humidity will fall into the teens
at times across southern Wyoming. Mixing and smoke dispersal will
range range from fair in some of the basins to good to very good in
some of the windier locations.