Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Grand Junction Colorado
1120 PM MDT Tuesday Aug 26 2014
issued at 938 PM MDT Tuesday Aug 26 2014
Southwestern Utah convection moves into southeastern Utah later this evening...but
will be encountering a more stable regime. Surface temperatures have
cooled significantly with readings in the upper 50s to middle 60s.
Storm Prediction Center mesoanalysis also show our air mass have less instability per
MLCAPE values. Strong storms over southwestern Utah will have weakened
significantly once this activity reaches southeastern Utah. Thus the
Flash Flood Watch was cancelled earlier this evening.
Showers with embedded storms have not ended as a vorticity lobe
rotates around the anomalous closed low along the Nevada/Utah
border. The band of moisture over The Four Corners gets lifted
northward and wraps around the upper low...expanding in the
process. This will result in widespread showers across NE Utah and
the northern 2/3rds western Colorado. Potential exists for significant precipitation
in the upslope region of the Uinta Mountains...the area northwest
of Vernal needs to monitored for any flood potential given near
saturated ground. Areal Flood Advisory already issued for eastern
Daggett County in the Jarvie ranch as gravel/dirt roads have been
reportedly washed out.
Upper low will be above the western slope Wednesday afternoon. May
need to consider this a Spring cold core system with higher
precipitable water content. The rain cooled air mass will have
stabilize the air mass...expect afternoon destabilization from middle
level cold advection. With lower freezing levels...some thunderstorms
will contain hail. Isolated large hail is possible (although the
wind shear profile is not favorable)...but the more likely
scenario is plenty of small hail with significant accumulation.
The cool wet August for the northern and central areas of the
forecast area continues for another day.
Update issued at 625 PM MDT Tuesday Aug 26 2014
Anomalous closed low will track toward western Colorado tonight and
will be overhead on Wednesday afternoon. Moisture surging
northeast from southern Utah and Arizona will eventually wrap around
the middle level circulation with precipitation more focused across
the north by Wednesday. Appears that much of the area will receive
showers with embedded storms on Wednesday. Will take another look
at the forecast package when the 00z hrrr arrives with possible
forecast updates for Wednesday.
Short term...(tonight through Wednesday night)
issued at 234 PM MDT Tuesday Aug 26 2014
Convection starting to get going this afternoon thanks to the
closed low over Utah...available moisture...upper level trough
rotating around low and upper level dynamic support. Model differences
continue with some models being more aggressive than others with
hrrr still doing pretty well picking up on most convection across
nm...UT...AZ...and Colorado. While some showers and storms are moving
across the County Warning Area...the hrrr shows the stronger convection entering
southeastern Utah this evening and this looks to be panning out as a line of
showers and storms has formed over central and S central Utah and
is tracking eastward. High specific humidities lending themselves
to some heavy rainers so the flash flood watches will remain in
Rain with some embedded storms will continue overnight as the
upper level low approaches the forecast area. The storms and
showers over the San Juans will shift northward as the night
progresses and best upper level support also shifts to the
north...still rotating around the low pressure. Snow levels are
expected to drop near 13k feet or so overnight with a light
dusting of snow possible for elevations above that. For
tomorrow...most precipitation will be up north as the low pressure starts
to fill and move over Utah and Colorado. However...as the low moves over
the County Warning Area during the day upper level dynamics will again play a
factor with more precipitation on tap for mainly the San Juans northward.
Temperatures will stay below normal through the period due to
precipitation and cloudiness.
Long term...(thursday through tuesday)
issued at 234 PM MDT Tuesday Aug 26 2014
The upper trough drops onto the eastern plains on Thursday with
the western most associated showers close to the Continental
Divide favoring northwest-facing slopes. These showers are expected to
end by Thursday evening.
A Flat Ridge tries to build in from the southwest on Friday.
Residual moisture will allow late-day storms to fire mainly over
the higher Colorado terrain. The ridge gets flattened on Saturday
as the next Gulf of Alaska trough digs into the Pacific northwest.
Saturday still looks like the warmest day of the week perhaps
hitting 90 degrees once again in Grand Junction and near 100 in
the southeast Utah lowlands.
The Pacific trough will progress through the northern rockies
Sunday into Monday. Its 100kt jet streak sweeps through southern
Wyoming. This may produce scattered storms across the northern
mountains Sunday with a few storms persisting into Monday. These storms
will be moving to the east at 45 miles per hour so strong winds are a threat.
Elsewhere Sunday and Monday will have breezy afternoon winds with
a slight cool down recovering somewhat on Tuesday as the trough
works across the northern plains.
Aviation...(for the 06z tafs through 06z Wednesday night)
issued at 1119 PM MDT Tuesday Aug 26 2014
Numerous showers expected with embedded thunderstorms through the
forecast period. The strongest storms will produce 4sm
+shra/+tsrags with ceilings lowering to below airfield minimums for
the mountain taf sites. Convective turbulence should be expected
with occasional mountain top obscurations. Some improvement is
expected across the far south on late Wednesday afternoon and
evening with a decreasing chance for -shra/-tsra.