Thursday, February 22, 2018

Looking Out To March 1st

Even though large-scale trough remains over the West, it appears that main impacts locally will continue to be chilly temperatures and several windy periods for rest of February.


Graphics here are all from 00 UTC runs of global models and are valid at 00 UTC on March 1st. There is reasonable consistency in the GEFS 500 mb mean and spaghetti plots (above) . The most uncertainty at that time for CONUS U.S is over the west half of country. But real heaping amounts of spaghetti are piled over the North Atlantic and Europe.

Three of the GEFS member forecasts, plus the operational GFS, are shown below, with the operational GFS at upper left. The details of the western short waves have fairly large differences. The most interesting for us is the forecast at lower left, which is trying to dig a short wave over the ocean, rather than keeping things mostly over land. This would increase chance for a moist system during first week of March.

The operational ECMWF forecast (at bottom) appears fairly similar to the bottom right forecast from the GEFS. The ECMWF also looks promising wrt digging a short wave over the ocean. We will wait to see whether March starts out wet.




Wednesday, February 21, 2018

Below Freezing Here/Next Short Wave


View from campus at a bit after 7:00 am MST this morning - note temperature of 39F. Here at house the low this morning was 29F - the first morning of February with a low of freezing or below.


Shown here are the 72-hour 500 mb forecasts from 00 UTC last evening that are valid at 5:00 pm Friday afternoon - GFS above and ECMWF below. Both forecasts are similar, with a significant short wave crossing Arizona again. This weather system will bring strong winds again to Arizona. However, it will be another moisture-starved system, with light showers or sprinkles likely across the metro area.



Tuesday, February 20, 2018

Quick Overview Of 19 February

Yesterday produced light showers across the metro area (ALERT 24-hour precipitation at 7:00 am MST this morning - below) late evening with a few spots getting a second round after midnight. Amounts were were mostly below 2/10 inch except in mountains. Here at house we had 0.06". Coverage was impressive with 90 to 100% of the network receiving measurable precipitation and some snow at highest elevations.

Winds were very strong across most of state with gusts of 35 to 45 mph common. Highest winds I noted this am were above 50 mp and included: 70 mph at 4-m telescope on Kitt Peak; Guthrie RAWS 64 mph; Winslow 63 mph; and Flagstaff 55 mph.


Monday, February 19, 2018

PW Stays High - Wind - Showers And Colder


A very unsettled day in store for Arizona today. At sunrise heavy clouds (from bright to dark) hung over the Catalinas.


The 06 UTC GEFS plumes from yesterday (above) for local PW were a total bust, and PW remained about constant through the day and night - as per GPS PW time-series from campus below that has end time of ~ 12 UTC this morning. The MIMIC TPW analysis for 13 UTC (second below) indicates higher PW values continuing over south-central Arizona, ahead of strong cold front that will cross state today.



Yesterdays showers produced some measurable precipitation over the mountains and parts of the metro area. Afternoon showers here produced 0.01". I noted 0.33" at Pioneer Airfield.



Very strong and cold 500 mb short-wave trough will be moving into Arizona by noon today (above from 06 UTC WRF-GFS on 5.4 km grid last). Ahead of the associated cold front there will be strong winds (below 06 UTC WRF-GFS 10-m winds forecast for 2:00 pm MST on 1.8 km grid). Note the very strong winds forecast on the down-wind side of many mountain ranges (i.e., purple shades). This early morning winds are gusting over 50 mph and some locations in norther and eastern Arizona.

With the front and residual moisture approaching there will continue to be showers around, although the WRF-GFS forecast keeps measurable amounts mostly on mountains and east of Tucson.



Sunday, February 18, 2018

Winds, Cooler, And Slight Chances For Showers


Nice start to day on Sunday morning, as per view to north from campus. There does appear to be a shallow layer of grunge hanging along the foothills.


Stong 500 mb short-wave is forecast to dig south-southeastward toward Arizona during next 60-hours (above is ECMWF forecast from 00 UTC 18 Feb is valid at 00 UTC on the 21st). However, the GEFS plumes from 06 UTC for PW at TUS (below) indicate that PW will plunge from high values this morning to very low values by the 21st. This would mean more moisture-starved weather systems for rest of month.



However, the MIMIC TPW analysis from 7:00 am MST this morning (above) offers a bit of hope for next 48-hours, as higher PW seems to be curling northward west of northern Baja - indicating that the falling PW maybe overdone for next 36-hours (worth watching to see how the GEFS forecast PW plays out). Below is 06 UTC WRF-GFS forecast from Atmo for total precipitation through 5:00 pm tomorrow afternoon. This forecast is certainly moisture-starved except at higher elevations.



Saturday, February 17, 2018

Some Corrections Re GEFS

I have found that even though the operational GFS is run at approximately 13 km resolution, the GEFS ensemble members are run at half a degree (~54 km), I assume that this is because of computing time constraints. I have not been able to determine whether the physics packages are exactly the same. Regardless, the operational GFS has higher resolution and thus captures terrain gradients better than the GEFS.

Further, the GEFS does no interpolation and assigns the nearest grid value to TUS. The map below was provided by Mike Crimmins back when we were first hashing around this issue.


The grid map for the GEFS ensemble members for Arizona (below - provided by Tracey Dorian of NWS EMC) shows the location of TUS as a red triangle. Map at bottom also shows this point. The grid point is exactly the same as the one south of airport as provided by Mike.

The distance between the closest grid point and the airport is about 15 km and the grid point surface elevation is about 136 ft higher than the ASOS site at airport. So there is only a small amount of upslope or downslope missing from the GEFS plumes for Tucson.






Overview Of Our Very Significant Event


Our unusual wet period has brought us a rare event - morning fog. The campus view above, at about 7:15 am MST, appears to have a visibility of about a mile or so and conditions are similar here at the house. It remains very mild with temperatures in the 50s F. After months of mostly dry sunshine, the past 3-4 days have been very pleasant - at least from my perspective. The south image at bottom from Kitt Peak shows clearing skies, but with fog in the valleys.


General event totals through 6:00 am this morning (above, from Pima County ALERT) indicate around 1.50 inches across the low-elevations of the network. The exception is from the south of the airport down to Amado, where amounts exceed  2.00 inches, with highest at Green Valley - 3.23 inches. Here at house the final, multi-day event and four periods of measurable rainfall left a total of 1.50" in the gauge. The event time series for precipitation at the airport (below) indicates 4 or 5 rain periods there.

I have been looking back at my logbook and can't find a cool season, multi-day event with this large of a total back through all of 2015. Didn't have energy or time to go into older logbooks.



The period of unsettled weather here will continue through rest of month, as per the global models. The Pacific ridge has been pushed westward to about 140 degrees (above - showing GFS analysis at 00 UTC last evening). The GEFS 500 mb average heights out at 168-hours (below, valid at 00 UTC on February 24th) indicates our period with digging short-waves coming south down east side of ridge will continue. This, of course, represents a very substantial change from the Fall and Winter pattern that has been keeping us warm and dry.




Friday, February 16, 2018

Friday Morning Rainfall Amounts


Ghostly telescope structures in the cloud at Kitt Peak this morning. Total event rainfall amounts to 7:00 am MST this morning across the ALERT network (below) show that the event has produced generally over an inch of rain - much higher amounts in the Catalinas and Rincons. The 0.23" at Rincon Creek and X-9 Ranch appears very low, and there may be problems at that site. The subtropical moisture plume did a direct hit to our area, resulting in rainfall amounts that considerably exceeded the forecasts.



This afternoon poses another dilemma. The 06 UTC plumes for QPF at TUS (above) indicates 100% chances for measurable rain at the airport this afternoon - timing is very consistent but amounts range from less than 0.10" to 0.80" - a very large spread at a very short lead time. The 06 UTC WRF-GFS forecast of composite radar echoes below is valid at 3:00 pm this afternoon and indicates another band of light showers across eastern Pima County. I expect that the amount at airport will be lower than both operational GFS (blue) and the mean GEFS amounts (heavy black). The moisture remains high, but a layer of middle-level dry air has moved across the area early this morning. That will tend to suppress shower activity unless more moist air replaces it, or sunshine breaks out and triggers deeper convective showers. Will be interesting to watch how the day evolves.

NOTE - my previous post about the GEFS and its grid point interpolations has some serious errors that I have uncovered, and I will report on latest findings this afternoon.



Thursday, February 15, 2018

Current Rain Amounts/Radar Data


Graphics shown here are from about 6:45 am MST to 7:00 am. I checked the ALERT network last evening at 5:00 pm, and amounts were small but coverage of measurable precipitation was already 100%. Light sprinkles began here around 10:45 am, but it was mid-afternoon before light rain began. The plot above shows amounts for past 24-hours across the network. Generally, amounts currently range from about half an inch to well over an inch - very large amounts indicated on the Catalinas. Have to admit that I haven't yet ventured outside to read our gauge, since the rain continues.



I thought that I'd take a look at some products from our KEMX radar data, but as processed at other places than the NWS webpage. Above and below are the reflective fields for tilts 1 through 4 from the College of Dupage weather page (link to the right). The base scan at top graphically shows the severe terrain blockage that affects our radar. The radar beam is blocked to the southwest all the way through the 4th tilt (second below). The blockage, especially for low-top non-thunderstorm events, significantly impacts the KEMX estimations of rainfall.

The higher tilts show a very distinct bright band circle for our current setting. The morning sounding indicates the top of the bright band (the ice to water melting zone) has its top just at or below 700 mb.
The bright band impacts composite radar displays, spreading out the apparent extent of reflectivites reaching 40 dBZ or so. The estimated rainfall will be biased toward high amounts when the data bins used to estimate rainfall rate happen to also be in the melting zone. A complicated situation that will result in overestimates when just the radar data are considered.

At the bottom is the current echo tops product - this indicates the precipitation is low-topped, reaching only to around 500 mb. A check of the CG flash data reveals absolutely no flashes in Arizona and northern Mexico during the event to current..

Final note: the CoD radar products indicate elevation tilts of 0.5, 1.5, 2.5, and 3.5 degrees. However, commercial radar analysis software (GR2Analyst) indicates the radar is operating in the VCP 212 mode - this scan strategy has 7 tilts between 0.5 and 4.0 degrees. The CoD products are either interpolated or using the nearest data to their fixed tilt elevations.





Wednesday, February 14, 2018

GEFS Plumes And Grid Points Used For Tucson

Question re this morning's post: Was there a consensus reached on the location or grid size used for the GEFS plumes? I enjoy looking at them to determine model-to-model variance but I don't find them to be representative of much more than that.



About a year ago we (myself and several readers) tried to determine where the 4 closest GFS grid points were relative to the airport. Presumably the four closest grid point forecasts are interpolated to yield the point forecasts for the airport. This proved to be a very frustrating exercise, since NWS technical documentation is sadly lacking important details, or much out-of-date. Mike Crimmins came up with the grid points shown above. However, when I examine them closely, they appear to be on the old GFS 27 km grid - current grid is 13 km. So I assume that the points that are interpolated to give the forecast for TUS are considerably closer to the airport than shown here. I just have been searching to try to find more detail - but alas just end up frustrated. It is important to remember that on a 13 km grid, the terrain is still grossly smoothed, so  that the real-world up and down slope flows are not accurately represented.

The GEFS plumes for QPF are the most unreliable forecasts, while other parameters (e.g., temperature, wind etc.) are more reliable. All of this makes it important to consider the 1.8 km grid forecasts from the Atmo WRF versions (even though these are deterministic forecasts driven by single model forecasts, as per the GFS version, the terrain effects are more accurately captured). The GEFS plume forecasts for the entire country can be found at:

http://www.emc.ncep.noaa.gov/gc_wmb/tdorian/gefs/EMCGEFSplumes.html

A number of more progressive NWS offices use ensemble model forecasts to predict QPF amounts probabilistically. Typically these forecasts indicate the likelihood of different precipitation thresholds occurring - an example would be 20% probability of rainfall greater than inch, etc. etc.

Regardless, it remains unclear to me exactly how the GEFS plumes are derived for any given point. If someone can direct me to links that explain the details of the process, I would certainly appreciate it.


Valentine's Day - Quick Update


Little time this morning allows just a very quick update. A totally overcast sky this morning - that's a big change in itself.

The 7:00 am MIMIC TPW analysis below shows the subtropical moisture plume evolving (since yesterday) basically as forecast by models for past week, with highest PW seemingly pointed right at southeastern Arizona.


The 06 UTC GEFS plumes above (for TUS QPF) show all model members forecasting measurable rainfall at the airport with main event beginning later today and then extending into Saturday morning the 17th. The large spread in times and amounts indicate that the moisture intrusion for the Pacific is being forecast somewhat differently by the various members.

The 06 UTC WRF-GFS forecast (driven by the operational version - blue above) shows a forecast of 0.80" at the airport. There is a substantial gradient in amounts, with over half an inch decrease from the airport out to central Pima County.

Rainfall here of more than 0.40" would be the most significant event since August 13th last year - good potential for the first ground-soaking rain since late summer.