Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Grand Junction Colorado
530 PM MDT sun Jul 5 2015
Short term...(tonight through Monday night)
issued at 306 PM MDT sun Jul 5 2015
Classic monsoon pattern featuring high pressure over West Texas
and low pressure off the California coast has brought deep
subtropical moisture to the forecast area. Precipitable water (pw)
as measured at 12z at kgjt was 1.29 inches...or nearly 2 times
normal. A shortwave trough moving northeastward across northeast
Utah and northwest Colorado sustained clouds and light showers
over the area during the early part of the day. Meanwhile...
thunderstorms began to develop over southeast Utah where clouds on
the western edge of the moisture plume had diminished.
Expect diurnally driven storms will continue through sunset with
slow movement and moist sub-cloud layer points to areas of heavy
rain and localized flooding...especially over the south where
insolation was favored given latest satellite trends. Meanwhile
..models indicated that the shortwave will be over The Divide by
00z/Mon. Limited insolation due to clouds combined with subsidence
on the back side of the trough axis should inhibit strength and
coverage of thunderstorms across the north this evening. Diurnal
cooling in the absence of forcing will cause showers and
thunderstorms to diminish later this evening for most areas.
However...models suggest another shortwave moving through the
northern rockies will brush the northern border of the forecast
area late tonight possibly sustaining a few nocturnal showers
through sunrise Monday.
Passage of the northern trough will cause flow to shift to the
west...temporarily ending the influx of moisture from the Mexican
plateau. As a result...models suggest precipitable water values will dip to near
0.80 of an inch on average. This level of moisture is not
inconsequential and is more than sufficient to generate scattered
to numerous showers and thunderstorms over the mountains with a
few storms moving over the adjacent valleys. Steering flow will
continue to be weak and resultant slow moving storms may bring
localized flash flooding. To varying degrees...models indicated a
weak shortwave moving overhead Monday night may sustain showers
and embedded thunderstorms well into the night.
Clouds and shower activity are expected to continue to moderate
high temperatures Monday while having an opposite affect on
Long term...(tuesday through sunday)
issued at 306 PM MDT sun Jul 5 2015
Zonal flow will continue into Tuesday with precipitable water values remaining
near 0.75 of an inch as the low remains off the California coast.
Diurnal warming will fuel another round of showers and
thunderstorms during the afternoon/evening time period. Models
hint at another eastward propagating shortwave trough moving over
the area Tuesday night which could sustain showers and embedded
thunderstorms late into the night.
Flow veers back to the southwest on Wednesday as the eastern
Pacific low pushes ashore over the central California coast late
in the day. Models hold the low quasi-stationary through Thursday.
Moisture changes little during this period so expect another
round of diurnally driven showers and thunderstorms both Wednesday
Storms likely to be more energetic...especially across the south...
on Friday as a 50-60 knots jet rounds the base of the Pacific low and
noses over The Four Corners region. Steering flow picks up which
will move storms along a better clip which...when combined with precipitable water
values falling below 0.6 of an inch...reduces the threat of flash
flooding. Models in agreement with drying continuing into the
weekend as the moisture plume is shifted eastward ahead of the
low. Regardless...expect a few storms to fire each afternoon as precipitable water
values hover near 0.40 to 0.50 of an inch.
Aviation...(for the 00z tafs through 00z Monday evening)
issued at 526 PM MDT sun Jul 5 2015
Showers and thunderstorms will continue across eastern Utah and
western Colorado through 06z tonight. Expect ils ceilings near storms
with locally heavy rain and reduced visibilities as well. Storms end
after 06z with VFR conditions expected through midday Monday.
Another round of thunderstorms develops after 18z Monday...with
locally heavy rain...small hail and gusty winds again expected.
Mountains will be obscured at times.
Short term...no layers
long term...no layers