Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Tucson Arizona
938 am MST Friday Apr 18 2014

Synopsis...a Pacific low pressure system will bring a chance of
showers and thunderstorms later this afternoon and especially
Saturday. Cooler temperatures will also occur today followed by a
warming trend early next week. Gusty winds will return to the area
Tuesday and Wednesday.


Discussion...forecast was updated to reflect quicker advance of
moisture resulting in thicker cloud cover this morning and slightly
better chances of measurable precipitation over the higher terrain
today. Still an outside chance of thunder late today in heat of the
day as per latest hrrr product. Thunderstorms...or light rain
showers/virga for that matter...could produce erratic wind gusts.
There is a better chance for such a scenario tomorrow with increased
instability nearer to the upper low.


Aviation...valid through 19/18z.
Upper level low pressure near the Southern California coast will
produce overcast cloud decks mostly above 12k feet above ground level today in Southeast
Arizona. Isolated-scattered -shra should develop this afternoon and isolated -tsra
not out of the question 18/22z-19/02z. Surface wind becoming
southerly/southwesterly 15 kts with gusts to near 25 kts this afternoon...perhaps
up to 35 kts near -shra/-tsra. Chances for -shra/-tsra are better
on Saturday. Aviation discussion not updated for taf amendments.


Fire weather...a weak upper level low pressure system will move
east across the area through Saturday. There appears to be enough
moisture to justify isolated to scattered showers this afternoon
through Saturday. There also appears to be enough instability to
support a slight chance of thunderstorms on Saturday...and cannot
completely rule it our for today. Rainfall amounts are expected to
be rather light for valley/desert locations today and Saturday...but
could see widespread amounts around a half inch or more for mountain
locations on Saturday. However...the main concern will be for the
potential of gusty and erratic winds associated with any virga or
thunderstorms that occurs.

A few showers could linger near the New Mexico border Sunday but dry
conditions should prevail through Wednesday. Expect normal diurnal
wind trends Sunday and Monday...with occasional gusts during the
afternoon hours due to daytime heating.

Critical fire weather conditions may return as early as
Tuesday...though more likely Wednesday when stronger winds are
forecast to occur. The strongest winds will be across
Greenlee...Graham and Cochise counties east of Tucson.


Previous discussion... /issued 240 am MST Friday Apr 18 2014/
satellite imagery this morning showed extensive cloud cover over southern
Arizona in advance of upper low that was off Southern California/northern Baja California CST.
Precipitable water values across the area this morning were around
0.40" with higher values near 0.65" over Southern California and southern Sonora
Mexico with even higher values up to 1" off Baja California spur. As precipitable water values
increase today...the atmosphere will be moistening from top down. So
lots of virga this afternoon with a chance for measurable rain in
the higher terrain. Inverted v soundings this afternoon will lead to
gusty and erratic winds near virga shafts. Highs today under the
thick cloud cover will be 7-12 degrees cooler than Thursday.

Tonight into Saturday...upper trough axis will move into western Arizona later
tonight then across the state on Saturday. Precipitable water values across the area
are forecast to be between 0.60" and 0.85" which is almost 2
Standard deviation above normal for April. Made some slight
adjustments to areal probability of precipitation this evening into Saturday morning. With
the trough axis over the area Saturday...have significantly increased
probability of precipitation in the mountains....with smaller increases across lower
elevations. I based this on latest runs of local WRF model which
hits Saturday afternoon pretty hard with localized heavy
rainers/isolated thunderstorms. Also increased quantitative precipitation forecast values in the
mountains but this will likely change with later model guidance.

Lingering showers Saturday night across eastern areas as trough axis
pushes on east. With plenty of middle-level moisture around on
Sunday...could see heat of the day showers develop over the higher
terrain. Otherwise it will be warmer Sunday with additional warming
Monday with upper ridge axis over the state.

Tuesday and Wednesday...breezy to windy conditions as strong upper
trough moves through the intermountain west with timing issues between
GFS/ECMWF. Will maintain continuity with previous forecast and have
Wednesday being the windiest day. Fire weather concerns are
addressed in the fire weather section below.


Twc watches/warnings/advisories...none.




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