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Area forecast discussion 
National Weather Service Riverton Wyoming
331 am MDT Friday Jul 25 2014

Short term...today through Sunday night

WV imagery showing flat upper ridge west/ high centered over eastern nm. Trough
that has flattened the ridge is running eastward along the Montana/California border.
The dirty portion of the ridge has now been mainly pushed southward into
northern Colorado. At the surface...low press to the east in western South Dakota...high press west and
over the Pacific northwest. Weak dry surface front cutting right through Wyoming from NE to
SW. Deep dry air has moved into the forecast area from the SW/west today. Upper high
over southwestern Continental U.S. Remains in place for the most part for the next few
days...just slowly trending westward through the forecast period.

Today will be cooler...with a deep dry atmosphere in place...with
virtually no chance for active convection except for possibly near
the Wyoming/Colorado border region. More significantly...increased fire danger
..including red flag warnings...Will continue across the southern and
eastern zones through 9 PM tonight. However...will remove the Cody
foothills (zone 276) from the red flag warnings today as this area
behind the front does not look to produce both the winds and low relative humidity
values needed. Otherwise...daytime temperatures will cool into the upper 80s
and lower 90s for most low elevations sites...60s and 70 in the
mountains. Min relative humidity values will range from the single digits to the
middle teens for most basins and valleys...20s in the mountains. Winds
by middle to late afternoon will blow out of the west 15 to 25 miles per hour from
central to southern Wyoming...including the mountains. The strongest
winds in the red flag areas look to remain under and south of the
developing low in southern Wyoming...mainly targeting the south central and
southern zones. Much of Fremont and Natrona counties will probably
not quite reach the wind criteria needed for the rfw...because of
position relative to the developing surface low...but it is too close to
not to keep the warning going and there will certainly be at least
isolated areas that do reach the min criteria. Still...all other
locations across the forecast area will be under enhanced fire danger for one
reason or another.

Sat and Sat night...flat dry zonal flow begins the transition to dry
northwest flow. Lee side low pressure moves off into the Central
Plains...with additional cooling following the passage of another
cool front east of The Divide...but slight warming west of The Divide
where the front will not be a presence. The surface p gradient should also
relax for most locations...with strongest winds of the day across
mainly Sweetwater County (fire zone 279) nearest low pressure to the
S in Colorado. Not going the extend the rfw for any zones
at this time...including zone 279 as it looks like winds may remain just
below rfw criteria...although min relative humidity values will certainly be low
again. The combination of the two will may not be there. Will
monitor further over the next 24 hours for a decision before expiration
later tonight.

Sunday...upper ridge rotates and amplifies above the
forecast area...transitioning from zonal to northwest flow...then to weak zonal
as axis aligns north/S through the region with heights and temperatures rising.
Warmer with highs well back into the middle 80s to near 90. With surface
high p now east of the County Warning Area and lower p to the SW...and only a modest at
best p gradient and weakening upper level flow as the ridge expands
amplifies and orients overhead...winds will not be an issue this
day. May be a very small chance for rain showers/thunderstorms and rain where low moisture
will rotate up from the SW in weak ring o fire...over the SW
mountains.

Long term...Monday through Friday

Blocking pattern sets up across North America during this period
with high amplitude ridge over the western Atlantic keeping longwave
trough over the eastern U.S. With upper ridge over the intermountain
west...upper trough across the eastern Pacific. Like an l.A.
Freeway at rush hour...little movement will occur with these
synoptic features.

However...monsoonal moisture's path will be unimpeded around the
periphery of the ridge...making a northward surge across the Great
Basin into SW Wyoming on Sunday night/Monday while return flow
around surface high pressure over the High Plains brings Gulf
moisture into Wyoming from the east...with this moisture converging
across the southern two-thirds of Wyoming by Tuesday with precipitable water
values of 1 inch or greater. So on Monday...initial plume of mainly
middle and high level moisture will bring an initial round of isolated
to widely scattered thunderstorms into the SW Monday morning/early
Monday afternoon with this activity expected to spread north and
east during the late afternoon and evening. As lower levels moisten
up on Tuesday expect scattered thunderstorms...some heavy rain
producers...across the south and central with less activity across
the north where there will be less moisture and instability to work
with. Forecast trends high temperatures on Tuesday across the south
several degrees cooler with expected cloud cover and convection
through the day.

On Wednesday and Thursday...both GFS and European model (ecmwf) show some suppression
of the Gulf moisture feed from Central High plains with plume of
moisture sandwiched along The Divide along and west of The Divide
with low level upslope flow...northwest flow aloft. Storm motion will be
southeast...so most activity is expected to develop and track nearly
parallel to the southern absaroka and Wind River ranges.

Medium-range models show some de-amplification...broadening of flow
across the Continental U.S. Friday. This will be a slow process that will
result in little or no change to pattern over The Rockies with plume
of moisture...the "ring of fire" of convection likely to remain over
the Great Basin and along the Wyoming Divide Friday and into the
weekend.

&&

Aviation.../12z issuance/

Generally clear sky with VFR conditions through 12z Saturday. Any
thunderstorms today will be suppressed vicinity and south of a kdgw-
krwl line. The surface wind will once again increase by middle
afternoon. West wind could be gusty...especially along the central
and northern mountain ranges of Wyoming. A dry cold front will
surge south across central Wyoming this evening...near a vicinity kcpr-klnd
line by 03z...with winds shifting north and then gradually
diminishing after 06z Saturday.

&&

Fire weather...

A red flag warning continues for portions of central and southern
Wyoming through tonight...mainly for the afternoon and evening
periods. Critical fire weather conditions have developed across
these areas where critically dry fuels also exist. Although high
temperatures will drop today over those the past few days...very
warm to hot daytime temperatures in the upper 80s and lower 90s will
combine with a drying atmosphere and very low relative humidity values ranging from
the single digits to the middle teens and increasing west winds. Most
of the enhanced fire weather conditions will be tied to another
relatively strong surface pressure gradient across the southern
forecast area that will also combine with decent middle/upper level
flow to give westerly wind gusts between 25 and 35 miles per hour in the
afternoon and early evening across most of the warned area. Similar
but weakening conditions will be present Saturday...although high
temperatures will be somewhat cooler again over the warned areas
behind another dry cool front...but will still remain warm and dry.
However...both the surface pressure gradient and upper level flow
should weaken just enough to keep winds from reaching red flag
conditions...while Haines index will generally fall between 4 and 5.
Fire danger will still be enhanced...especially across zone
279.

&&

Riw watches/warnings/advisories...
red flag warning until 9 PM MDT this evening
wyz279-280-283-285-287-300.

&&

$$

Short term...Braun
long term...aem
aviation...aem
fire weather...Braun

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