Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Riverton Wyoming
230 am MDT Wednesday Jul 29 2015
Short term...today through Friday night
A warming and drying trend begins today as high pressure builds
into the region. Expect to gain a few degrees each day with the
area of low humidity spreading from the southern third of Wyoming
today...further north on Thursday and Friday with much of the
state expecting single digit relative humidities by the end of the
week. The good news is that winds will be relatively light
underneath the ridge but the heat and low humidity will bake the
vegetation beneath increasing fire danger as this warm and dry
trend continues. There is a remote chance of some thunderstorm
activity on Friday as upslope flow along the i25 corridor draws in
some meager middle level moisture...but probability of precipitation were kept quiet due to
the low probability of enough moisture to kick off convection.
Long term...Saturday through Wednesday
Synopsis...upper level high pressure centered near The Four
Corners area will bring above normal temperatures through the
period. Dry weather will prevail across most of the area Saturday.
Middle and high level monsoonal moisture will bring slight chances of
thunderstorms to a large portion of the area by Monday.
Discussion...medium range models remain in overall agreement with
strong upper level high pressure setting up near The Four Corners
region...becoming the dominant...persistent weather feature through
the first few days of August. Western and central Wyoming will be
positioned slightly downstream of ridge axis through Tuesday with west-
northwest flow aloft and 500 mb heights in the 590-594dm range...700 mb temperatures
averaging around 18c in the SW to 15c in the NE. This will bring
high temperatures in the lower to middle 90s across the central
basins.. with mostly 80s in the western valleys. Some middle and high
level monsoonal moisture will begin to drift around the northern
periphery of The Four Corners upper high over the weekend...bringing
possibly a few storms to the northwest mountains on Saturday...with isolated
thunderstorms across the northern half of the area on Sunday. Most
of the area will be under the 'ring of fire' on the periphery of the
upper high Monday and Tuesday. Lower levels of the atmosphere are
expected to remain dry so any thunderstorms will be mainly
dry...producing more wind than rain. Fuels will continue to dry
out over the next few days...and will be primed for lightning
ignitions and new wildfire starts in this scenario. Upper ridge
axis is forecast to translate the area on Wednesday bringing a SW
flow across the area and more shortwaves coming in off the Pacific.
This may bring better chances of afternoon/evening thunderstorms to the
northwest...though confidence is low with models progression of
large-scale flow 8 days out.
VFR conditions are expected through 12z Thursday. Only a few fair
weather cumulus are expected through Wednesday night. Winds will be
light with mainly diurnal variations at the usual locations such as
kjac...kpna and kbpi.
A warming and drying trend will continue through the weekend.
Light winds will be found through the end of the
week... however...min rhs will be in the single digits to middle
teens each day across the south and areas east of The Divide...and
in the 20s across the west. No significant precipitation is
expected through the end of the work week...however a brief period
of upslope flow may bring in some middle level moisture from the
plains...increasing instability on Friday enough for a Haines of 6
this will be a fairly efficient drying period for fuels...so when
this ridge finally does break down fire activity could be pretty
vigorous. Looking way out in the models...there could be a few
waves moving over the ridge with the first to break it down
significantly on Monday afternoon...at which time dry
thunderstorms and gusty winds will be possible.