Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Grand Junction Colorado
141 PM MDT sun Aug 2 2015
issued at 139 PM MDT sun Aug 2 2015
A tight little cyclone was rotating into SW Mesa County this
afternoon. This feature was associated with a 5-3 deformation
zone moving north at 5-10 miles per hour with widespread rain and isolated
storms. This deformation feature is captured well in the nam12 and
will lift over the I-70 corridor this afternoon then into the
piceance basin late afternoon and evening. Only light to moderate
showers are expected under this feature. South of this line
convection will become more intense through the afternoon and
evening. A stronger rotation was working northward into southeast Utah on
the nose of the 35kt jet. This will potentially produce heavy rain
showers favoring southeast Utah and far SW Colorado into this evening.
Storms will also become widespread over the San Juan Mountains into
this evening and coordinated with the Pueblo office to extend the
Flash Flood Watch until 3am MDT.
Update issued at 823 am MDT sun Aug 2 2015
As mentioned in the short term forecast...Doppler radar is showing
cyclonic circulation over SW Colorado this morning. The SW quarter
of the forecast area received some good rainfall in the last 24
hours. Half to three quarters of an inch was reported at several
sites in southeast Utah up to the Tavaputs plateau. This helps set the
stage for potential flash flooding today and through the evening.
Storm motion will be slow or non-existent through this evening.
The 12z NAM shows a weak 35kt S-north oriented jet lifting into southeast
Utah this afternoon. This is a bit further west than the 06z run.
The favored location for heavy rain looks to favor southeast Utah this
morning then spreads across light snow shower westcentral Colorado this
afternoon and evening. The morning sounding here at Grand Junction
resolved 1.00 inch precipitation water which is right in between the
forecast values from the GFS (0.86) and NAM (1.1). This indicates
the models may not be resolving important details so the broad
watch looks like a good product.
Short term...(today through monday)
issued at 400 am MDT sun Aug 2 2015
Today is on track to be a wet day across much of eastern Utah and
western Colorado...though areas north of the Tavaputs/bookcliffs
region are not likely to get soaked until the weather system
lifting north through the region today reaches the northern areas tonight
and into Monday. A 500 mb closed low develops over southern Nevada
today with an elongated vorticity maximum stretching east into southwest
Colorado. As this energy moves through the already moisture-primed
Four Corners region a lot of activity is expected today from
south to north. The rap13 and NAM forecast models indicate a weak
circulation near The Four Corners this morning which indicates
this energy will provide plenty of dynamics for heavy rain. The
energy slowly moves north through the day and into the overnight
hours. Though showers and thunderstorms are expected to somewhat
diminish during the overnight hours...by Monday morning a well
defined circulation looks to present itself over eastern Utah.
This circulation will provide lift to the antecedent moisture and
create a busy day across NE Utah and northwest Colorado.
Biggest challenge at this point is how far the heaviest rain will
reach today and then where it will fire back up on Monday. Looking
at vertical profiles...steering winds will remain quite low today
and vary based on the developing weak circulations embedded aloft.
Middle level winds do increase on Monday so flash flooding will not
likely be as big of an issue but we will need to watch the
increasing vertical shear for other potential issues on
Monday...including hail and strong winds from thunderstorms.
Long term...(monday night through saturday)
issued at 400 am MDT sun Aug 2 2015
Short wave will be sliding downstream by Monday night...but there
is a secondary but weaker upper level wave that lifts northeast
across NE Utah and western Colorado Tuesday afternoon. Combined with
some convective potential...showers/storms will pop again over the
western Colorado high country. Precipitable water will be under an
inch and trending lower...so showers/storms will not have the
Subtropical high over the southwestern Continental U.S. Recovers and dominate the
pattern...at least until Friday. Mostly dry weather is expected
Wednesday and Thursday...although a late Day Mountain storm can
not be completely ruled out.
The high core repositions itself a tad east and allows for the
monsoonal moisture plume to surge north beginning on
Friday...although the deeper moisture may not arrive until
Saturday. The trend will be for increasing showers/storms from the
south at the end of the week.
Aviation...(for the 18z tafs through 18z Monday noon)
issued at 823 am MDT sun Aug 2 2015
Showers and storms will expand across the entire region this
afternoon...showers becoming numerous south of I-70. The strongest
cells will produce 4sm +tsra ceilings bkn030. Ils ceilings and
obscurations will be common at mountain taf sites as
showers/storms move overhead. Showers will continue overnight.
With a short wave passing through Utah and Colorado this afternoon
and tonight...convective cells will be longer lived and mergers
may occur to enhance +shra with frequent lightning. Scattered
storms and showers will continue overnight with a 40 percent
chance of -shra/-tsra at kvel kcny kgjt after 06z. Other taf
sites best chances of -tsra/rain showers occur before 06z.
Colorado...Flash Flood Watch through late tonight for coz006>012-014-
Utah...Flash Flood Watch through late tonight for utz022-027>029.