Reading a NWS area forecast discussion

by Tim Vasquez
©2000 Weather Graphics Technologies

Forecast discussions (also known as a FPUS, FPUS3, AFD, or SFD) are issued by every National Weather Service office, and you can use them to get a glimpse into the mind of the duty forecaster. Forecast discussions can be obtained from databases like IWIN and other sources.

AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SPRINGFIELD MO 200 PM CST MON JAN 14 2001 This is the standard heading. Pretty standard fare.
DILEMMA DU JOUR: DO WE NEED A HEADLINE FOR TUESDAY NIGHT? A headline is a term for a "banner" that is often placed at the top of the National Weather Service forecast during major weather events to emphasize the importance to media weathercasters and other users.
RIDGE OF HIGH PRESSURE ACROSS THE CENTRAL AND SOUTHERN PLAINS HAD PRODUCED A SUNNY BUT COOL DAY ACROSS THE FORECAST AREA. STUBBORN AREA OF LOW CLOUDS CONTINUED TO SKIRT THE FAR NORTHEASTERN EDGE OF THE FORECAST AREA...WITH HIGH CLOUDS STREAMING INTO SOUTHWESTERN SECTIONS ON A 130 KNOT JET. A ridge of high pressure (which can be either at the surface or aloft) is often associated with good weather. Typically there is subsidence (sinking) within the ridge. The forecaster mentions stubborn low clouds, indicating that they might have played havoc with the forecast for that area, and that high clouds are riding in swiftly on a strong 130 knot (150 mph) jet stream.
MODELS STILL HAVE THEIR DIFFERENCES WITH THE NEXT SYSTEM TO AFFECT THE OZARKS: AVN REMAINS THE STRONGEST AND FARTHEST NORTHWEST WHILE THE ETA/MESOETA ARE MORE PROGRESSIVE AND WEAKER. LEANING TOWARD THE ETA/MESOETA SOLUTION TODAY...AS THE AVN MAY BE TRYING TO ABSORB THE WAVE INTO THE NORTHERN STREAM SYSTEM TOO FAST. The forecaster has looked at the computer-generated model forecasts and figures that there are differences between the different types of models. The AVN (also known as GSM and MRF) show stronger systems (larger upper-level waves and deeper surface systems) with most of the region's weather features biased toward the northwest, while the ETA solutions are more progressive (shallow waves and shallow surface systems, which tend to move rapidly with the upper-level flow). The forecaster prefers the ETA solution, however the reason did not make sense (at least to me).
CLOUDS WILL CONTINUE TO INCREASE THIS AFTERNOON/TONIGHT AS SHORT WAVE CIRCULATES AROUND THE DEVELOPING LOW IN THE DESERT SOUTHWEST AND MOVES INTO THE SOUTHERN PLAINS TUESDAY MORNING. The proximity of the upper-level low to the southwest will generate clouds for the area because it will create dynamic lift and will be the focus for embedded upper-level disturbances that rotate around it (see the 500 mb chart). These will increase as the upper-level low approaches.
TWO WAVES OF LIFT EXPECTED ACROSS THE REGION: FIRST WILL COME AS A COUPLED JET SIGNATURE MOVES ACROSS THE OZARKS TUESDAY AFTERNOON. SECOND WILL BE TUESDAY NIGHT WHEN ACTUAL SHORT WAVE MOVES THROUGH. ODD THING ABOUT THIS EVENT IS THAT ALL OF THE LIFT IS ABOVE 700 MB. It appears here that the forecaster expects a right-rear quadrant of a polar jet will couple with the left-front quadrant of a subtropical jet, which can produce enhanced upward motion. This will be followed by lift from an upper-level disturbance. The forecaster notices from the model output that all of the lift occurs above 10,000 ft (700 mb), which is indicative of considerable dry air in the lower troposphere or possible shortfalls in the model solutions.
CURRENT FORECAST HAS ONE TO TWO INCHES OF SNOW ACCUMULATION AND SEE NO NEED TO DEVIATE FROM THIS IDEA. DO NOT BELIEVE THAT ANY HEADLINE IS NEEDED AT THIS POINT...BUT LATER SHIFTS WILL HAVE TO REEVALUATE. Pretty obvious wording. There is some concern that headlines will need to be added to the forecasts, but this will be left to later forecasting shifts.
EXTENDED FORECAST LOOKS COOL BUT DRY FOR THE MOST PART. THINGS GET UGLY FRIDAY AS NORTHERN AND SOUTHERN STREAM SYSTEMS MEET OVER THE MIDDLE MISSISSIPPI VALLEY BEFORE CREATING A STRONG STORM FOR THE EAST COAST. NOT MUCH IN THE WAY OF PRECIPITATION EXPECTED...WITH NORTHERN STREAM SYSTEMS STARVED FOR MOISTURE AND GULF RETURN LIMITED BY PRECIPITATION ALONG THE COAST. WILL ADD A CHANCE OF LIGHT SNOW FOR FRIDAY WITH HIGHEST CHANCES IN SOUTHEAST SECTIONS. A jet stream segment across the southern US and the northern US as well as two upper-level disturbances will come in close proximity with each other (see 500 mb chart). This indicates that the atmosphere is highly baroclinic (contains strong air mass contrasts) and is a breeding ground for strong weather systems to develop. Obviously the threat area will be downstream from these two regions: the east coast. The sentence about "not much in the way of precipitation" applies only to the Kansas City region, and was written since the southern system will be ingesting much of the moisture that could have been destined for the Kansas City area, and the northern system due to the pattern and its northerly position will be dry. However the forecaster feels there is enough of a threat from the proximity of the systems to add in a chance for snow.
THANKS TO TED AT STL FOR COORDINATION THIS AFTERNOON The forecasters occasionally coordinate their predictions with other offices; in this case it was the St. Louis office.
PRELIMINARY GUIDANCE FOR COORDINATION PURPOSES ONLY SGF 023 035 023 030 020 / 5 40 70 20 JLN 024 036 023 031 022 / 5 70 70 10 UNO 025 038 025 032 023 / 0 30 70 30 VIH 022 034 022 029 018 / 0 30 70 30 This is coded temperature and precipitation forecasts. The first line reads Springfield, low tonight 23, high tomorrow 35, low next night 23, high next day 30, low next night 20. Chance of precipitation 5% tonight, 40% tomorrow, 70% the next night, and 20% the following day.
.SGF... KS...NONE MO...NONE SUTTON No warnings or watches are in effect for the region. The forecaster's last name is Sutton.
FXUS64 KOUN 112159 COR
351 PM CST MON DEC 11 2000
This is the bulletin header. It was prepared by a forecaster at the NWS in Norman, Oklahoma, and was issued at 3:51 pm on December 11, 2000.
COLDEST AIR IN ALMOST FOUR YEARS HAS TIGHTENED ITS GRIP ON THE SOUTHERN PLAINS...WITH DAYTIME TEMPERATURES HOLDING IN THE TEENS ACROSS CENTRAL AND NORTHERN OKLAHOMA. AS CAA ABATES...AND CENTER OF SURFACE HIGH APPROACHES...STRATUS SHOULD ERODE...ALLOWING FOR A STRONG RADITIONAL COOLING NIGHT...WITH TEMPS EXPECTED TO DIP INTO THE SINGLE DIGITS ACROSS MOST OF THE CWA. WOULDN'T BE SURPRISED TO SEE A FEW LOCATIONS IN NOTHERN OKLAHOMA DIP BELOW ZERO. Cold high pressure is moving in, and the downward motion within the high pressure area will allow the low stratus clouds to erode. This will create clear skies, which allows the ground to radiate heat into outer space. This will result in strong cooling across the county warning area (CWA).
CLOUDS WILL ONCE AGAIN INCREASE TOMORROW ACROSS THE AREA AHEAD OF FAST MOVING SHORTWAVE CURRENTLY OFF THE CALIFORNIA COAST. A short wave is a somewhat ambiguous term for a disturbance in the upper-level flow that produces upward motion (i.e. it tends to produce clouds and rain). There is one that will be approaching from the southern California coast, and it will likely produce clouds and upward motion.
FORECAST THEN GETS EXTREMELY TRICKY...WITH POSSIBILITY OF WINTER PRECIP EVENT LOOMING TOMORROW AFTERNOON/EVENING. STUCK CLOSELY WITH ETA SOLUTION FOR THIS PACKAGE...AS AVN APPEARS TO PLACE QPF TOO FAR TO THE NORTH...GIVEN FORECAST WAA FIELDS...AND TRACK OF SHORTWAVE. The ETA model (equivalent to the Greek letter "eta") is one of the best forecasting models in use today. Here it forecasted winter precipitation. The AVN, also known as the GSM and MRF, is an older but updated model that is more accurate in large-scale rather than small-scale depictions. It placed the quantitative precipitation forecast (QPF) of precipitation too far north. The forecaster assumed that the best warm air advection would be further to the south (over Texas), which implies the ETA run is doing a better job.
AVN HAS BEEN CONSISTENT FROM RUN-TO-RUN IN DEVELOPING SLIGHTLY STRONGER SHORTWAVE THAN ETA. TRACKS BETWEEN THE TWO MODELS ARE VERY SIMILAR...SO FEEL SOMEWHAT CONFIDENT IN WHERE THE HEAVIEST PRECIP WILL OCCUR...BUT AM LESS CONFIDENT CONCERNING HOW HEAVY IT WILL BE. Back to the AVN/GSM/MRF, which is great with large-scale patterns. It developed a stronger upper-air disturbance, which intrigues the forecaster. The AVN and ETA show a similar forecast track for the disturbance, but there is some question how strong it will be.
WAA BEGINS TO INCREASE IN MID LEVELS TOMORROW AFTERNOON...MAINLY ACROSS NORTH TEXAS/SOUTHERN OKLAHOMA IN ADVANCE OF LEAD SHORTWAVE. THEREFORE...WILL MAINTAIN POPS TO ACCOUNT FOR THIS. Warm air advection (WAA) throughout the middle level of the atmosphere (about 10,000 to 20,000 ft, or 700 to 500 mb), is expected to increase. This supports upward vertical motion. Therefore the forecaster will maintain significant probability of precipitation numbers (POPs) in his forecast to account for this.
WITH MAIN WAVE KICKING OUT TOMORROW NIGHT...WILL INCREASE POPS FURTHER...AGAIN MAINLY ACROSS SOUTHERN ZONES. ETA FORECAST SOUNDINGS INDICATE THAT ANY PRECIP WILL LIKELY BE IN THE FORM OF SNOW. Warm air advection in combination with lift from a short wave disturbance is a fairly substantial indicator of upward motion. Therefore as the warm air advection phases with the short wave, the forecaster is further boosting the odds of precipitation. The forecast soundings (vertical temperature profiles) indicate snow, which likely means that the entire column of air is subfreezing and will produce ice particles.
WITH INTENSITY OF FORCING...ANTICIPATE THE POSSIBILITY OF SOME SIGNIFICANT SNOWFALL TOTALS...SO WILL ISSUE WINTER STORM WATCH... MAINLY FOR NORTH TEXAS AND PORTIONS OF SOUTHERN OKLAHOMA. Although the model forecasts aren't clear on the strength of the short wave, the forecaster feels there will be enough warm advection and/or lift from the short wave to produce strong lift and thus significant precipitation. With this and the fact that the forecast sounding shows subfreezing temperatures, snow is the best bet.
WITH SPEED OF SHORTWAVE...WINTRY PRECIP SHOULD NOT LINGER TOO LONG INTO WEDNESDAY...AND WILL BEGIN TO CLEAR THINGS OUT WED AND WED NIGHT. THURSDAY AND FRIDAY WILL SEE SOME RECOVERY FROM THE ARCTIC AIRMASS...WITH HIGH TEMPS EXPECTED TO WARM INTO THE 40S BY FRIDAY. The disturbance will rapidly take the bad weather with it, and for reasons not specified (probably high pressure and a ridge aloft), clear skies are forecast with warming.
MAJOR CHANGES INTRODUCED FOR THE WEEKEND FROM PREVIOUS PACKAGES...AS MRF...CANADIAN...AND EUROPEAN ALL FAIRLY CONSISTENT IN BRINGING ANOTHER SHOT OF ARCTIC AIR INTO THE AREA. WILL ALSO INCLUDE SLIGHT POPS FOR SATURDAY FOR ANY LIGHT SNOW/FREEZING DRIZZLE DEVELOPING BEHIND ARCTIC FRONT. The forecaster has seen the latest AVN/GSM/MRF run, along with Canadian and European (ECMWF) models, and this shows more arctic air coming into the area. The forecaster feels that more precipitation is a possibility with it, probably because of mid-level warm air advection and/or a short wave disturbance.
JL FCSTID = 4 The forecaster's initials are JL, and his office ID is 4.
OKC 4 22 17 25 / 0 30 50 30 HBR 8 21 16 26 / 0 50 50 30 SPS 11 26 20 28 / 0 60 70 40 GAG 1 19 15 24 / 0 30 30 30 PNC 0 18 16 24 / 0 0 30 30 DUA 11 25 20 28 / 0 60 60 40 These are quantitative forecast numbers. The first group is the min and max temperatures, and the second group is the probability of precipitation in percent. Translated, for Oklahoma City (OKC), the next day's forecast low is 4 deg F, with the high 22, and the following day the low will be 17 with a high of 25. The next day's chance of precipitation is 0% in the morning and 30% in the evening. The following day, the chances are 50% in the morning and 30% in the evening.
.OUN... OK...WINTER STORM WATCH TUE AFTN AND TUE NIGHT ZONES OKZ036>039-041-043-045>048-050>052. TX...WINTER STORM WATCH TUE AFTN AND TUE NIGHT ZONES TXZ083>090. This group indicates what watches and advisories are in effect, along with the zone numbers that they are assigned to.

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