Ramblings of a Graduate Student

It’s been a while since I’ve done a forecast post of any sorts, so tonight I thought I’d give (most) everyone bad news: a very hot heatwave is potentially in the works for next week. The image above (and denoted below) is from this morning’s run of the Global Forecast System (GFS) Model and is valid for Monday evening. It depicts a very large subtropical ridge (high) over much of the United States. This ridge will bring with it very warm temperatures across a large portion of the United States, and not much potential for rain. The main storm track lifts north into the far northern United States and southern Canada, so cold fronts will be hard to come by.

Not shown here, but the GFS is hinting at the possibility of a tropical wave of some sort to develop off the Atlantic coast by late next week. It will certainly bear watching!

Well, I’ve made it to day 200; that’s 200 consecutive days of blogging. I should get a gold star or a cookie or something, right?!?

Anyways, a relatively quiet weather pattern persists across the United States tonight. A large, subtropical ridge is located across the southern United States, helping to keep the heat locked in place across the southern half of the US. To the north of this ridge, mid-level flow is a bit stronger on the periphery of the ridge. Embedded within this stronger flow, several minor shortwave troughs are racing through the flow, helping to initiate daily bouts of convection acorss Nebraska and Iowa. This convection then develops into an MCS overnight and pushes eastward and/or southward during the subsequent day.

However, I wanted to turn attention toward the tropics. A large upper-level trough is moving through the Gulf of Mexico currently; it looks like a giant “6″ in the water vapor image above. This upper-level trough is increasing shear throughout the Gulf of Mexico, which will help inhibit any tropical storm development in the near term.

Turning attention toward the Puerto Rico area, a large area of tropical showers and thunderstorms are ongoing just north of the island. The National Hurricane Center (NHC) currently gives this tropical wave a 20% chance of developing into a tropical depression/storm during the next 48 hours. This is partly the result of strong shear co-located with the tropical wave, owing to the mid-level low currently located just to the north and east of the tropical wave. Model forecasts show the shear weakening atop the tropical wave in the coming days, which will give the tropical wave a chance to develop into a tropical depression/storm, just about the time the wave moves into the Gulf of Mexico. It’s still a long ways off, but certainly is of more concern than anything else over the last week or two.

The cold front mentioned last night has pushed south and east into the region that was experiencing the extreme heat wave. This cold front will bring an increased chance of thunderstorms and slightly cooler temperatures to much of the central United States. This slight cool down and increased chance of thunderstorms will be enough to end the Excessive Heat Warnings and Heat Advisories that were in effect yesterday.

In the longer term, expect the heat to return early next week as the upper-level ridge once again builds back into the central United States. However, at this time, it doesn’t appear to be as strong. (But that could change…)

Tonight’s images come from the Climate Prediction Center (CPC) and are forecasts for the next 6-10 days (above) and 8-14 days (below). These are forecasts of temperature relative to normal. As I mentioned in the last few days, the ridge in the east that was responsible for the extremely hot temperatures along the east coast last week is now redeveloping over the central United States. As such, expect above normal temperatures in the near term.

I’ve been working on a project with a deadline coming up so I haven’t been able to devote as much time to the nightly blog as I would have liked. Tonight is no exception…

The heavy rain (and cooler temperatures) that have been centered over the southern plains the past week will be shifting to the east. People in the east should expect a cooler and wetter pattern the next several days. This can be attributed to the large high pressure that was centered over this region last week redeveloping over the south-central United States. This will bring a return to hot and humid conditions in places that were wet last week and wet conditions to places that were hot and dry last week.

As I mentioned last night, tropical systems release large quantities of moisture into the troposphere. One measure of the amount of moisture in the troposphere is “precipitable water (PW)”. In a simple sense, PW can be thought of as the amount of water that would be collected in a rain gauge if all the moisture in the atmosphere above the rain gauge were to instantly fall as rain. In this definition, we make the assumption that no new moisture would be added to the atmosphere.

Based on this simple definition, it would be easy to assume that the PW value is the maximum amount of rain that could fall in a given environment. Unfortunately, this is not the case. The atmosphere is continually evolving; this is particularly the case whenever it is precipitating. For example, while a thunderstorm is producing rain over a certain area, it is drawing air in from the surrounding environment. The air from the surrounding environment is acting to replenish the moisture that is lost by the precipitation. Thus, it is inaccurate to use the PW value as a maximum amount of rainfall. Instead, what the PW value can be used to ascertain areas where heavy rain is possible – which would be wherever high values of PW exist.

In the sounding above, taken 00 UTC 2 July 2010 (7 PM CDT 1 July 2010) in Corpus Christi, TX, there is some question about the accuracy of the PW value listed in the bottom part of the image (3″; sever lines beneath the word “Parcel”). In any event, this is a very moist sounding and is representative of a very tropical airmass left behind former hurricane Alex.

At the same time, a mid-to-upper-level trough (indicated by the “dip” in the orange lines) is moving into the western United States. Located to the east of this trough is a ridge over the eastern United States (indicated by the “circle” located over the Memphis, TN area). In response to the west coast trough and east coast ridge winds are southerly. This is slowly drawing the aforementioned tropical airmass north into the central plains. As the large scale ascent associated with the trough (indicated by the thunderstorms over Montana this evening) moves east and begins to interact with the tropical moisture spreading northward, the stage will be set for very heavy rainfall. Couple this with a strong ridge located over the eastern United States, which will result in a very slow eastward progression of the west coast trough, a prolonged, heavy rain event will be possible somewhere in the central United States.

A very active weather pattern has resulted in repeat rounds of showers and thunderstorms for some across the eastern two-thirds of the US, while others remain locked into a major heat wave. The reason for this is the position of two mid-tropospheric weather “features”: one a ridge and the other a trough.

The ridge (denoted by the blue H) is positioned over the Red River between Oklahoma and Texas. Underneath this ridge, the air is sinking, which causes it to warm and dry (in the mid-levels, not the surface). This results in very hot, dry weather conditions for those stuck underneath. This pattern is often referred to as the “Ring of Fire” because it is very hot in the middle, and “ringed” by showers and thunderstorms. Just look at a national radar mosaic and you’ll see this is the case tonight!

The trough (not denoted) is located across the western United States. This large trough is helping to draw moisture northward into the central United States, which results in a hot, humid mess under the ridge. This large trough also has a series of smaller troughs (denoted by red circles) and jet streaks (yellow fill) moving through it, each bringing a source of lift. Couple this lift with the moisture streaming northward, and a threat of severe weather exists. In all actuality, this is why there has been severe weather almost every day along the periphery of the ridge. This pattern doesn’t look to change anytime soon, so expect the change of severe weather to continue for the next few days.

Lastly, in this kind of pattern, it is extremely difficult for a tropical system to actually make landfall across the northern and eastern Gulf of Mexico. This is because the steering flow around the ridge would tend to keep the tropical systems moving westward, and not bring them north into the Gulf of Mexico. An exception to this might be far south Texas, which is on the southern edge of the ridge. This is good news for beach goers and the oil spill relief efforts. However, these kinds of patterns can change quickly, so this doesn’t mean the chance of tropical systems in the Gulf will remain low all season. Coastal residents should make sure they have a hurricane preparedness plan ready to enact should a tropical storm or hurricane threaten.