Phoenix Gets a Pounding

Someone was going to get a beating from the ongoing monsoon surge and right now it is Phoenix.  Observations from the Phoenix International Airport (KPHX) show that 3.09" of rain has fallen since midnight.

Source: NWS

The NWS has issued flash flood warnings for much of the area.

Source: NWS

The 1200 UTC (0600 MDT) analysis below shows the mix of airstreams feeding the storm system, known as a mesoscale convective system, this morning.  Potentially contributing to this event are moisture sources associated with Tropical Cyclone Norbert, a Gulf of California surge, and even the Gulf of Mexico, although the juiciest air appears to be originating from the Pacific and the Gulf of California.

The potential for thunderstorms that could be locally heavy exists across much of Utah today through tomorrow.  Keep an eye to the sky.  NWS forecasts at http://www.wrh.noaa.gov/slc/.

Major Monsoon Surge Underway

You can say "yeah, but it's a dry heat" all you want today, but the reality is that it will be quite humid (by local standards) in many parts of the desert southwest today and tomorrow.

This morning (1500 UTC/0900 MDT), the decaying circulation of former Hurricane Norbert (now Tropical Storm Norbert) sit off the Baja coast.  A major slug of moisture (integrated precipitable water contours below illustrate the total water vapor in the atmosphere) to Norbert's northeast is now pushing into the lower Colorado River Basin and the surrounding region.

The corresponding higher-resolution 3-hour forecast of integrated precipitable water and surface wind from the NAM shows the complex stew of water vapor that will be influencing the southwest the next couple of days.  Yeah, integrated precipitable water values are high around Norbert, but there is also a related surge of Gulf of California moisture into the lower Colorado River Valley and moisture originating to the east near the Gulf of Mexico flowing westward as well.  

Current dewpoints over the southwest are not totally outrageous, but are enough to make you sweat.  How about 67ºF in San Diego, 76ºF at Imperial County Airport, 69ºF at Fort Yuma, 63ºF in Boulder City, and up to 67ºF in the Phoenix metro area.  

Source: MesoWest

The NAM forecast loop below shows the impressive push of moisture today and tomorrow from the Gulf of California up the lower Colorado River basin into southern Nevada and Utah.  Along the Wasatch Front, we have a chance of thunderstorms through the period.  I watered my lawn yesterday, so that ensures a good deluge.  

Flash flood watches have been issued by the NWS for much of the region experiencing the surge (green fill below).  

Source: NWS

And, in case you are wondering from previous, there will be no hurricane for San Diego.  Looks like after substantial weakening, the upper-level remnants of Norbert will track across northern Baja on Tuesday and Wednesday (It's unclear if the circulation center at the surface will ever move onshore).  Mother Nature will not go Beatrice Prior and choose the GFS solution.  

Divergent Forecasts Continue

Like a child coming of age in Divergent, as a forecaster one choice can transform you and today that choice is do I join the GFS faction?

The GFS continues to be a dauntless like outlier amongst the various models with regards to the evolution of Hurricane Norbert.  It continues to hold on to a solution in which the remnants of Norbert make landfall near San Diego on Monday.

Source: Wundermap

This contrasts with the gold-standard ECMWF solution

Source: Wundermap

and even the, gasp, NAM.

Source: Wundermap

I'd show the Canadian, but it's pretty much a repeat of the ECMWF and NAM with Norbert tracking more slowly to the northwest and well off the SoCal coast on Monday.

The pros at the National Hurricane Center are clearly leaning toward the non-GFS solution and keep Norbert well south of San Diego through Monday.

Like Beatrice Prior in Divergent chosing Dauntless, perhaps Mother Nature will "chose" the GFS faction.  More likely she will stick with the safe choice and go with the ECMWF–NAM faction. Why the GFS is such an outlier is an interesting problem worthy of investigation, whether it gets it right or wrong.

BTW, if you are interested in reading about rare hurricane impacts on southern California, see The San Diego Hurricane of 2 October 1958 by Michael Chenoweth and Christopher Lansea.

Tropical–Extratropical Interactions Causing Forecast Challenges

The most recent satellite loop shows that trouble is brewing in the tropics in the form of Hurricane Norbert and a potent slugs of high precipitable water (a measure of the total water vapor content of the atmosphere) over both the eastern Pacific and the Gulf of Mexico.

1200 UTC 2 Sep – 1200 UTC 4 Sep 2014 IR satellite imagery with GFS analyzed precipitable water contours every 5 mm

Model forecasts for the next several days, however, differ depending on how each model handles an approaching upper-level trough in the midlatitude (i.e., extratropical) westerlies.  The upper-level trough digs in the GFS, eventually leading to large scale southwesterly flow over the eastern Pacific and southwest U.S.  As a result, Norbert tracks quickly northward and eventually curves toward SoCal coast, potentially bringing heavy rains to the area on Monday.  If the GFS forecast were to verify, gusty winds would likely accompany the Norbert remnants as they make landfall, but winds would likely be below tropical storm force.  

0600 UTC 4 Sep GFS forecast valid 2100 UTC 8 Sep 2014.  Source: Wundermap.

In contrast, the upper-level trough in the EC remains a bit further north and the flow over the eastern Pacific is weaker.  As a result, Norbert moves more slowly and remains well to the south on Monday. 

0000 UTC 4 Sep ECMWF forecast valid 2100 UTC 8 Sep 2014.  Source: Wundermap.

There are also differences in the forecast for Utah.  With the deeper trough, the GFS taps into a slug of moisture and brings a potent monsoon surge into the state (note the widespread precipitation in the GFS forecast above).  In contrast, the surge is weaker in the ECMWF.

These are the sorts of divergent forecasts that one can get during weak flow situations with interactions between tropical and extratropical systems.  Time will tell if this will be another victory for the ECMWF, an upset by the GFS, or some sort of draw (i.e., Mother Nature splits the difference).  For predictions like this, forecaster's often need a friend.

How Much Will It Snow This Winter?

The silly season is upon us with every Tom, Dick, and Larry putting together some sort of guess as to what this winter will be like.  One of my favorites is below.  Wet and "muggy" for the Pac Northwest.  Yes, beware Washington your dewpoints could hit the mid-to-high 50s!  

Source: http://weatheradvance.com/

As far as Utah is concerned, you know where I stand.  There is very little skill in seasonal snowfall predictions for the Wasatch Mountains.  We covered this ad nauseam last year (see Outlook for the 2013–14 ski season).  The basic problem is that the main game in town for seasonal prediction is the El Nino/Southern Oscillation and our snowfall simply doesn't correlate very well with El Nino, La Nina or "No Nino" (a.k.a., neutral).

Source: Jeff Massey

You can throw the alphabet soup of circulation anomalies like PDO, AMO, or NAO at the problem too and similarly there's simply not much correlation or forecast skill to be had for our neck of the woods.

Seasonal climate models?  Well, here's the forecasts for February 2015.  Take your pick.  For northern Utah some are wet (green-blue) some are dry (brown-red).  In virtual-reality land, you can have whatever February you want.

Source: NCEP

So, after careful consideration, here's my winter outlook for the central Wasatch Mountains.

It's a pretty good bet, at least in the upper Cottonwoods where a bad year is usually good for 400" of snow above 8500 feet.  

August Precipitation in Context

August was a great month in Salt Lake City.  I haven't run my sprinklers in quite a while.  If only every summer could by like this.  How unusual was it?  Let's have a look.

Although we often present historical weather data in chronological order (i.e., ordered by year), it can be helpful to instead present the data in ascending order, as I have done below for the August precipitation in Salt Lake City from 1928–2014.

As highlighted above, the median and average August precipitation in Salt Lake City for the period of record are 0.54" and 0.81", respectively.  The median represents the value at which 50% of the years are drier and 50% of the years are wetter and it is lower than the average, but the average is skewed higher by a small number of wet Augusts.   Thus, a curious aspect of the August precipitation climatology in Salt Lake City is that there is a 64% chance of precipitation at or below the average and only a 36% chance of precipitation above average (it is not uncommon for precipitation distributions to be skewed in this way, especially in arid regions).  So, if you want to make some money off your friends, bet them each year that the August precipitation will be below average.  You'll win about 64% of the time and slowly but surely build up a war chest of cash that you can blow in Vegas where the odds are certainly not in your favor.

So now let's look at 2014.  It seemed really wet and it was with 1.77" falling at the Salt Lake City airport.  That's more than double the average and more than three times the median.  It was also the 11th wettest August on record.

However, although it was a relatively wet August, the historical record shows that it can be far wetter.  In 1968, 3.66" of rain fell at the Salt Lake City international airport, more than double what fell this year (compare blue August 1968 accumulation with green August 2014 graph below).  In fact, 1968 was a very wet month across much of the region.

Given the spotty nature of monsoon rainfall, ideally one needs to look at a larger number of stations to gain a better context for the entire Wasatch Front.  Perhaps someone out there can dig through the data and add comments about accumulations at other sites.  Concentrate on sites with good data back to 1968 since that seems to be a critical year.