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Area forecast discussion 
National Weather Service Grand Junction Colorado
119 PM MDT Tuesday may 5 2015

issued at 0116 PM MDT Tuesday may 5 2015

Thunderstorm development has remained quiet thus far this
afternoon. A few strikes in the eastern uintas and Tavaputs in
eastern Utah as well as one northeast of Craig Colorado and
a couple more just north of Parachute. The thinking yesterday was
that this would be more of a stratiform event which is what has
materialized so far. However...there is a convective element to
this system and will continue to see thunderstorms develop around the
periphery of the closed low through the evening.

Update issued at 1125 am MDT Tuesday may 5 2015

Rain has spread north into east central Utah and west central
Colorado. Convection is beginning on target with the best
thunderstorms expected south of the main cloud shield over the 4
corners. Heavy rain...small hail...and frequent lightning are
expected with thunderstorms late this morning and through this
evening. Expecting showers and thunderstorms to become widespread
by the middle of this afternoon as the upper low moves northeast
throughout the day.


Short term...(today through wednesday)
issued at 346 am MDT Tuesday may 5 2015

Models remain in good agreement with plenty of precipitation in the
forecast over the next few days. Short range models...hrrr and
rap...have performed fairly well this evening with precipitation
locations and if this holds true...precipitation will continue for the
San Juans and pick up for the southern valleys overnight. All of
this is in response to an area of low pressure currently centered
over Arizona and cyclonic flow that is pulling in moisture from
the south allowing precipitable waters to reach values over 200% of normal.
Pieces of energy will eject from the low and be the focus of
precipitation today until the low pressure becomes an open wave and moves
newrd across our County Warning Area and itself becomes the focus of precipitation.

While some thunderstorms are expected and remain in the cover...Low Cape and marginal instability will
likely bring more stratiform rain than convective. Despite all
that...any peaks of sun that break through will likely set off
some storms that will produce some heavy rain. Expect precipitation to
get going around noon...if not a little before...and continue
possibly through Wednesday morning. The heaviest precipitation looks to
favor the central mountains and areas north of I-70 where best qg ascent is
located as the upper level wave moves across. That's not to say
that other areas won't see precipitation because much of the County Warning Area
will...including valley locations...but the heaviest remains
favored up north. Some runoff concerns are certainly possible if
the models hold true which is looking more and more likely. Some
snow is also expected at elevations above 10k feet though roads
will likely remain too warm for much accumulation.

Not much rest Wednesday as the next upper level shortwave moves
across the area bringing another round of showers and storms with
higher terrain favored at this time. While precipitable waters will start
dropping...values will still remain above normal. Temperatures will run
cooler than normal due to cloud cover and precipitation.

Long term...(wednesday night through monday)
issued at 346 am MDT Tuesday may 5 2015

Drying begins Wednesday night as a closed low along the West Coast
digs southward during the night. In response...flow becomes more
southerly tapping into drier air off of the Baja California coast. The region
will see precipitable water (pw) fall from near 0.50 of an inch to
around 0.25 of an inch on Thursday afternoon. As a result...expect
any moist convection Thursday afternoon/evening will be high based
and anchored to higher terrain. Similar conditions are expected
Friday...though GFS hints that moisture will begin to increase a
bit as the low center drops over Southern California for a slight
uptick in shower and thunderstorm activity. This is less apparent
in the European model (ecmwf) solution contrast to the GFS solution...
begins moving the low eastward to the Arizona/Nevada/Utah triple
point by 00z/Sat.

Model differences persist through the weekend with the European model (ecmwf)
remaining the more progressive of the two solutions. Despite
disparities...both solutions indicated that the forecast area is
headed toward another cooler and unsettled period. Given the
variability of the low/S track over the past several model runs
will continue to defer to the blended solution which keeps
mountains in the chance category with lower probability of precipitation in the valleys.

Consensus even harder to come by Monday as the European model (ecmwf) builds a
shortwave ridge over the area...having kicked the low off to the
Great Lakes region. In contrast...the low drifts northward over
the High Plains in the GFS solution with a shortwave trough
sliding eastward across the region to begin next week. Predictably
..the ec solution points to quiet and warmer conditions while the
GFS points to a wetter and cooler day. Given model spread there is
no way to confidently side with either solution so will stay with
the blend for now.


Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1125 am MDT Tuesday may 5 2015

Rain has spread north into east central Utah and west central
Colorado. Thunderstorms are beginning to develop mainly north and
south of the main cloud shield though the entire region has an
elevated threat of thunderstorms today from 18z to 03z. Mountain
obscurations will occur intermittently throughout today. Outflow
winds from 25 to 30 miles per hour are possible in the vicinity of stronger
storm cells...though the biggest impact will be potential MVFR ceilings
and visible from stronger cells. Showers will decrease later tonight
but will have to keep an eye out for developing br/fog tonight across
eastern Utah and western Colorado. Precipitation is expected to
pick up again between 06z and 12z from south to north tonight and
Wednesday morning.


Gjt watches/warnings/advisories...


short term...tgr
long layers

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