Houston is located on the southeastern coast of Texas on the Gulf of Mexico. It is not an accident that it was chosen as the first forecast city for the WxChallenge for early October-- it's still hurricane season! Perhaps the organizers of the WxChallenge are refusing to give up on this pretty pathetic tropical season so we have to admire their spirit! Houston, TX is also a really important city to forecast for because it is a major shipping port (one of the busiest in the country). As Joe Sienkiewicz of the Ocean Prediction Center once said, "Only two things travel by air: people and fish. Everything else travels by ship." Also, Houston is home to the second highest number of Fortune 500 companies just behind our beloved NYC! Therefore, if you were to work for a private forecasting firm, I'm pretty sure you'd have some customers who cared a lot about Houston. (Source: Wikipedia is amazing)
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Time to stalk the airport! KHOU is William P. Hobby Airport with a WMO ID of 72244. The METAR location page of the Automated Surface Observing System (ASOS) that will be used to verify your forecast actually has images of the station from each direction (images are found directly below the map). This is the best airport stalking that we've been able to do since I started this blog last year! So you can see that this location is flat, in unimpeded (no real topography) proximity to the Gulf of Mexico and more closely Galveston Bay and pretty developed (more urban than not). So warm, moist air is likely constantly coming off of the Gulf given southerly and southeasterly flow and drier air will likely be advected by the mean flow from most other directions. We'll definitely keep that in mind.
A good question to ask before looking at the current weather pattern is what conditions are typically expected at KHOU? A quick way to familiarize yourself with the climatology of a station is to look up the climate normals which for KHOU are found here. The mean maximum daily temperature from the period of 1981-2010 for the months of September and October is in the mid-80's and the minimum is in the upper-60's. October is also one of the more wetter months for Houston with the exception of July which is during the height of the convective season. Temperatures and precipitation can always occur outside of the climate envelope, especially if a huge ridge of high pressure develops over the central US (above normal temps), a deep trough (below normal temps), or a tropical system crashes into the coast (way above normal precipitation).
Large-scale Pattern:
I suspect that forecasting for Houston, TX will be messy because there are three main forecasting challenges: mesoscale boundaries, diurnal convection, and tropical systems....
- A mesoscale boundary is a smaller boundary than a larger-scale (synoptic-scale) front and can form from the outflow of previous convection or just along some weakly convergent winds. If there is sufficient instability and I doubt moisture will be an issue, then there will be forcing for precipitation.
-Diurnal convection is an issue because given the absense of a strong synoptic-scale player, daytime heating of the surface can allow for any moist air near Houston to rise given enough instability to form storms.
-As I mentioned before, it is still the season for tropical activity and the Gulf of Mexico is sufficiently warm and convection has been consistently developing especially with the penetration of cold fronts into the Tropics that allows for the generation of vorticity (spin needed to organize systems) if the shear is not too detrimental.
A look at forecast 500-hPa heights according to the GFS shows that the general trend in heights seems to be relatively zonal in nature so diurnal convection may be the primary forecasting challenge during our first week of forecasting. Depending on the speed of the propagation of the low pressure system forecast to develop lee of the Rockies by the end of the week, there may be a cold frontal passage (cold FROPA) to deal with. And of course, we can't ignore any tropical activity.
Summary of what to watch out for:
- Small-scale (mesoscale) boundaries that may provide forcing for precipitation and changes in temperature
- Zoom-in and look around this version of the Weather Prediction Center (WPC) surface analysis to get a handle on what's going on that day and therefore what still may be an active player around 06 UTC.
- Cold fronts: heavy precipitation and rapid temperature changes.
- Tropical cyclones or general areas of tropical convection spinning around the Gulf.
- Check out the satellite the evening before to see if there are any clusters of convection that perhaps the National Hurricane Center (NHC) isn't too worried about but you should be! (Keep in mind that convection tends to weaken at night given its diurnal nature even over warm ocean waters so check the afternoon before to see what's happening in the southwestern Gulf.)
- Another good tropical website is the U. of Wisconsin-Madison CIMMS page with regional real-time products found below the TC-specific products (if there are even any active TCs, *sigh*).
- Trust the experts and read the NHC Atlantic Discussion, especially the Gulf of Mexico section.
Tips/Comments for Forecasting in General:
- Check out the current weather first via satellite, radar, surface observations, METAR, etc. You have to understand the present before you can look to the future.
- Get a feel for general pattern from NAM, GFS, etc. (available from here and here, for example). Identify the general pattern of low and high surface pressure systems and their progression, temperature advection at 850 mb, moisture at 700 mb, vorticity at 500 mb and the general jet structure at 300 mb. Make sure you are focusing your attention on Houston.
- Look at forecast model soundings for the period for KHOU.
- How are the models evolving in time? Check out d(model)/dt plots like this one that focuses on the GFS.
- Compare your idea for values with MOS (model output statistics). Keep in mind these are instantaneous output values and you can always have values above/below that between output times.
- Read the Houston/Galveston WFO forecast discussion for KHOU. These are the experts for that location, afterall!
Tips/Comments for Forecasting for High/Low temperature (Fahrenheit):
- Recall such basics as nighttime radiative cooling and daytime solar radiative heating, both of which are affected by cloud cover and fog.
- Frontal passages can affect your temperature forecast. Pay attention to the forecast timing of such frontal passages.
- Given quiet synoptic players, the evening dew point can act as a rough guess for the nighttime low (some forecasters use this as a first guess when they think the model output statistics (MOS) is bonkers). Therefore the wind direction may be a good indicator of humidity which may affect the nighttime temperatures,
- i.e. southeasterly winds should be more moist which means a higher dew point which means a higher nighttime temperature and northwesterly winds will advect drier air which means a lower dew point which means the possibility for lower nighttime temperatures.
Tips/Comments for Forecasting Wind Speed (knots):
- NWS outputs wind speed in mph so if you are comparing your forecast value to theirs keep that in mind. Otherwise the models output wind speed (WSP) in knots.
- Look through METAR or surface obs for wind direction and temperature changes. For example, westerlies may bring warmer continental air towards Houston for some synoptic situations.
Tips/Comments for Forecasting Precipitation (inches):
- Take a look at some higher-resolution models (e.g. WRF)
- What does the WPC show?
- Tropical activity?!? (Part of me wants there to be while another part of me doesn't at all.)
- Take a look at some GFS model-derived standardized anomalies of precipitable water (PWAT) to get a handle on any significant movement of the forecast of dry air or very moist air.
- Take a look at some ensemble data (e.g. SREF (South Central option) or for a zoomed-in national look try here.)
- Most products you will be looking at will output the QPF in inches already so no conversion is necessary.
Alright guys- grab your cowboy hats and boots and let's try to lasso up some great forecasts! It should be a challenging forecast city so it makes the perfect first one of the 2013-2014 contest! If you ever have any questions please feel free to stop by my office in Challenger 121. I have candy. That is all. Good luck with KHOU and let's go Team SBU!!!
