Two Decent Fronts Moving In

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A lot going on this AM, including a couple of chances for rainfall and some much cooler weather on tap for the weekend. All of this has been covered over the past couple of days but our weather is going to be more interesting in the short term and after a summer drought that is showing some positive signs of easing significantly…that’s a good thing for us weather watchers.

Before I get to all that, props to one of our FB friends this AM about alerting me to what’s going on towards the northern Gulf Of Mexico. There is a mass of storms down there that appear to have a broad weak circulation attached to them. this is drifting towards the waters of the Gulf Of Mexico and may indeed turn into some sort of tropical depression and potentially something a minimal tropical storm (Nadine?). The only reason I bring this up is that the storms were formed from what was Isaac. You can see it by looking at the 5-day water vapor satellite loop. It’s not really the remnants of the former hurricane, but a by-product of the storms that were generated from Isaac. Anyway IF you look at the loop you can somewhat see this happening.

The next item was brought to my attention from one of my colleagues at the NWS in Pleasant Hill. Starting today the GFS model, which has been beyond terrible forecasting surface conditions all summer long. For those of you who are into looking at the models on a somewhat daily basis you know how worthless this model has been for awhile. Well along comes a model fix. Here is some information about the whys and whatfors.

From: Tim McClung
Science Plans Branch Chief
Office of Science and Technology

Subject: Change in Land Surface Model in Global Forecast
System and Associated Cool and Moist Bias in Near
Surface Temperature and Moisture Fields

Beginning with the 1200 Coordinated Universal Time (UTC) model
run on Wednesday, September 5, 2012, the National Centers for
Environmental Prediction (NCEP) Central Operations will implement
a fix to the Land Surface Model used to drive the Global Forecast
System (GFS). This correction is expected to improve the cool
and moist bias in the near surface air temperature and moisture
fields during the warm season.

Starting in mid-June 2012, NCEP confirmed a problem with the GFS
near-surface temperature and moisture simulations. The model was
not verifying in the late afternoon over the central United
States when drought conditions existed. Specifically, users noted
a significant 2m cold and wet bias in both the MOS and GFS
gridded products. The NCEP Environmental Modeling Center (EMC)
traced the problem to a look-up table used in the land surface
scheme that modulates evapotranspiration based on vegetation type
and root zone depth. Current settings allowed for excessive
transpiration and plant-extraction of soil moisture from deeper
soil layers, which caused the lower atmospheric boundary layer to
become too moist and cool.

Not to get on a tangent but this is one of the many reasons why, when you read about any story that talks about long range forecast models that I really don’t get too worked up about the potential accuracy about the models themselves. Whether it be a model for the next 60 days…90 days 1 year or 20 or 30 years. It’s very difficult for models to be accurate past a few days, and it shows how a minor tweak int eh equations/data can create some large errors in model output.

OK with that said lets take a look at where we are right now…there are all sorts of little features on the map this AM…the main one for tonight and Thursday’s weather is a front that is moving through the Plains states now. The air behind the front is somewhat cooler but certainly drier.

The black dashed lines are little weird wind shifts/trofs that are dotted through the area.

The front should come through this afternoon, but ahead of the front the winds are switching more towards the west. This will not really help the rain chances today as this reduces the “convergence” in the lower part of the atmosphere. There may be an opportunity for some scattered storms to develop this afternoon, mainly SE of the metro. It’s something that will be watching as the day moves along.

The more significant front with the Canadian air is due in on Friday. Ahead of this front moist air will try and return towards the region tomorrow evening and may spark renewed convection. That will be one opportunity for rainfall. Then as the main front moves in on Friday¬† Here is the surface map for Friday Morning…note the cold front moving into the area. Rain should be developing near and even behind the front as the front undercuts the returning Gulf moisture that should be in the areas as well.

Eventually behind this front dewpoiints by Saturday should drop into the 40s. With such dry air in place and High Pressure nearby bring the light winds and clear skies, the potential is there for lows on Sunday AM to tank. We’ve been forecasting 47 degrees for the last couple of days and that continues today. FYI…the record low on Sunday is 46 set in 2008.

Finally getting preliminary reports of a large earthquake near the coastline of Costa Rico.
Preliminary data indicates a 7.9 magnitude quake. I have no reports of damage as of this writing but that would be the 2nd largest quake of 2012 and 7.6 magnitude quakes can do a lot of damage depending on the population. There is now tsunami warnings for the west coasts of Central America.

We’ll have more coverage of this as needed on the blog and on FOX 4


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