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jgf

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About jgf

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  • Four Letter Airport Code For Weather Obs (Such as KDCA)
    KHPN
  • Location:
    Connecticut
  • Interests
    Offshore Sailing and Racing
    Navigation
  1. Copied below is today's WPC Tropical Discussion (which is not the same as the Tropical Weather Discussion issued by the NHC..).., and as you can see it is dominated by discussion of a "pre-frontal shear line". I know what "shear" is - i studied mechanics in graduate school - and I have this definition from the AMS: "Shear Line: In meteorology, a line or narrow zone across which there is an abrupt change in the horizontal wind component parallel to this line; a line of maximum horizontal wind shear." and also this from Wikepedia: "Stationary fronts will either dissipate after several days or devolve into shear lines, but can change into a cold or warm front if conditions aloft change causing a driving of one air mass or the other. Stationary fronts are marked on weather maps with alternating red half-circles and blue spikes pointing in opposite directions, indicating no significant movement. When stationary fronts become smaller in scale, degenerating to a narrow zone where wind direction changes over a short distance, they become known as shear lines.[29] If the shear line becomes active with thunderstorms, it may support formation of a tropical storm or a regeneration of the feature back into a stationary front. A shear line is depicted as a line of red dots and dashes.[13]" This shear line is not depicted on any surface analysis or surface forecast that I have seen. They seem to have a pretty good handle on where it is, and where it will be in the future... i have attached a surface analysis below the discussion.., So, I have a few questions: what data or observations, or model results, are they using to determine the existence of a shear line? would it be expressed in surface winds.., or winds higher up.., or both? How can i see it myself? FXCA20 KWBC 261149PMDCA TROPICAL DISCUSSION - INTERNATIONAL DESKSNWS WEATHER PREDICTION CENTER COLLEGE PARK MD648 AM EST FRI JAN 26 2018PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JAN 26/06UTC: BUILDING POLAR RIDGE OVER THE WESTERN ATLANTIC/EASTERN USA ISDRIVING A POLAR FRONT ACROSS BERMUDA-THE BAHAMAS EARLY THISMORNING. THIS TRIGGERED A PREFRONTAL SHEAR LINE THAT IS SURGINGACROSS THE DOMINICAN REPUBLIC. STRONG BOUNDARY LAYER WINDSACCOMPANY THIS FEATURE...WITH SHIP REPORTS NEAR THE TURKS ANDCAICOS SHOWING WINDS OF 25-30KT. THIS IS ENHANCING LOW LEVELMOISTURE CONVERGENCE ALONG THE SHEAR LINE...WITH SATELLITE DERIVEDPWAT ANALYSIS SHOWING MOISTURE POOLING ACROSS NORTHERN HISPANIOLA.HOWEVER...PERSISTENT MID LEVEL RIDGE AND ASSOCIATED TRADE WINDSCAP CONTINUES TO INHIBIT DEEP CONVECTION IN ASSOCIATION WITH THISFEATURE. AHEAD OF THE SHEAR LINE...A FRESH NORTHEASTERLY FLOW ANDA PWAT MINIMA ENVELOP THE FORECAST AREA.THE SURFACE FRONT IS TO SAG JUST NORTH OF THE FORECAST AREA EARLYON SATURDAY MORNING...WHILE TRAILING ACROSS THE BAHAMAS TO THEFLORIDA PENINSULA. BOUNDARY IS TO REMAIN NEARLY STATIONARY THROUGHEARLY NEXT WEEK...WHEN IT IS EXPECTED TO FRONTOLIZE. THEPREFRONTAL SHEAR LINE MOVES ACROSS THE MONA PASSAGE TO NORTHWESTPUERTO RICO THIS MORNING...PULLING ACROSS SAN JUAN BY 18-21 UTCTODAY. THIS WILL BE ACCOMPANIED BY A 20-30KT WIND SURGE...COLDADVECTION AND A LOW LEVEL CONVERGENT PATTERN. THE SHEAR LINEENTERING THE SAN JUAN METRO AREA WILL COINCIDE WITH PERIOD OF MAXHEATING...ENHANCING CONVECTIVE INSTABILITY AS THIS FEATURE CROSSESTHE ISLAND. THE SHEAR LINE IS THEN FORECAST TO MEANDER BETWEEN THEVIRGIN ISLES-EASTERN PUERTO RICO THROUGH SATURDAY MORNING...MOVINGFARTHER EAST ACROSS THE ISLAND CHAIN LATER IN THE DAY. AS IT PULLSAWAY AND RIDGE STRENGTHENS OVER THE ATLANTIC THE LOW/MID LEVELWINDS ARE TO GRADUALLY VEER TO THE EAST DURING THE WEEKEND. STRONGBOUNDARY LAYER WINDS ARE TO PERSIST THROUGH MONDAY...TO CONTINUEFAVORING A COOL ADVECTIVE PATTERN AND GENERATION OF TRADE WINDSHOWERS ACROSS THE VIRGIN ISLES INTO PUERTO RICO.UNDER THE INFLUENCE OF THE WIND SURGE/SHEAR LINE CONVERGENCE ANDASSOCIATED COLD ADVECTIVE PATTERN...EXPECTING A MORE SEASONALPATTERN OF TRADE WIND SHOWERS TO AFFECT PUERTO RICO AND THE VIRGINISLES DURING THE NEXT FEW DAYS. FURTHERMORE... THE STRONGEREASTERLY TRADES ARE TO ALSO FAVOR GENERATION OF STREAMERS THAT ARETO CONVERGE ACROSS VIEQUES...CULEBRA AND EASTERN PORTIONS OFPUERTO RICO. THESE WILL COMBINE WITH PERSISTENT COOL ADVECTIVEPATTERN AND MEANDERING SHEAR LINE TO LIKELY FAVOR MODERATE TOHEAVY RAINFALL AMOUNTS ACROSS THE VIRGIN ISLES AND EASTERN PUERTORICO/SAN JUAN METRO AREA THIS AFTERNOON THROUGH SATURDAY MORNING.LIGHT TO MODERATE RAINFALL AMOUNTS ARE THEN EXPECTED THROUGHOUTTHE WEEKEND/EARLY NEXT WEEK...WITH MOST ACTIVE CLUSTERING ACROSSTHE EASTERN THIRD OF PUERTO RICO...WITH LOCALIZED MAXIMA OVERSIERRA DE LUQUILLO/EL YUNQUE.DAVISON/GALVEZ...WPC (USA) $$
  2. Hurricane Maria

    The SailFlow private sensor on Culebrita survived the passage recording gusts of ~140mph... the sensor is on the light house which is the highest point on the (tiny) island - i've attached a photo i took from the light house a few years ago
  3. Major Hurricane Irma

    12Z GFS with WaveWatch III significant wave height in meters 18m - probably the biggest i have ever seen it forecast...
  4. Major Hurricane Irma

    Significant Wave Height forecast (meters) from the NOAA WaveWatch III model almost 14m in the center... OPC is saying 50ft possible
  5. Major Hurricane Irma

    The School in L'Orient.., would be pretty low (pun intended..) on my list of places to shelter from a hurricane passing just north of the island... i can't really think off hand of any large public buildings that are at high elevation, but many houses/villas are i guess i would probably go to this party and hope for the best! edit - for those who don't know.., even though it says "Guadeloupe".., it is in Gustavia, St. barth, so just a few miles south of the track- St barth is in the french department of guadeloupe, which includes all the french islands in the caribbean
  6. Major Hurricane Irma

    i travel there pretty regularly too - while i agree with what you say, i think they will fare better than any other island would if it got hit with a cat 5 eyewall - as they might... it is basically a 1st world place - people don't live in shacks...
  7. Can't do Calc: What are my options?

    my very first post in the thread addressed the issue of insufficient proficiency in algebra yes, of course it's critical to have a good working knowledge of algebra before taking even a basic calculus class what i said in that post was that you need to go back to whatever level your weakness starts at, and learn from there - that might even be pre-algebra stuff - fractions, decimals, geometry, and so on... if you do that - and it might take a year or two - i am confident that you can reach a level of preparation that will enable you to succeed in basic calculus class based on your previous posts, i am sure you won't agree, so let's agree to disagree
  8. Can't do Calc: What are my options?

    I went to undergraduate school at a pretty good "highly competitive", but not great, smaller liberal arts college, with quite good science departments and very good engineering departments. like a lot of schools, that school had two tracks for first-year calculus. They had an easier track which was basically one year long - 1 semester of differential calc, and one semester of integral calc. This was the "cookbook" class - it was mostly taken by humanities majors, as well as some science/econ majors who wanted a BA rather than a BS. I maintain that pretty much anyone with high school math through algebra could succeed in this calc sequence - i am not sure it even involved trig. I certainly don't agree that it was necessary to "already know the material" to get say a B. nearly all homework and test questions could be solved by following the steps outlined in the example problems. The other track was much harder (different text books), and consisted of 3 semesters - differential, integral, and multivariable. this was mostly math and engineering students, as well as science/econ majors who wanted a BS. This sequence was not cookbook, and i was challenged by it. in this track, there were proofs, and it was common to see questions on exams that were not exactly like anything in the homework. Because i was an idiot and had no clue what i wanted to do.., i ended up taking both! I will note that I am old enough that I was in college before it was common for students to have had calculus in high school. Today, many students going to "highly competitive" colleges have had some level of calculus in high school. My kid's high school offered two levels of first year calc - roughly equivalent to the levels i outlined above, as well as a pretty advanced multivariable class.., and a linear algebra class. the kids taking multivariable and linear algebra, had all had the harder first year of calc in their junior year of high school!
  9. An aesthetic point

    if tropical cyclones are in part a reflection of the heat imbalance between equatorial regions and polar regions.., wouldn't the observation that the poles are warming more rapidly than equatorial regions argue for fewer cyclones?
  10. so I understand that in a basic sense, when i look at the super res velocity in radarscope, i am seeing the magnitude of the vector component of wind parallel to the radius from the radar location - at the altitude of the return but when there is no rain.., what is actually being measured? here is KOKX (Upton NY) saturday august 28 at about 11:25am it's a bright sunny day - for the most part, as indicated by the reflectivity the velocity plot is indicating relative movement with respect to the radar my question is: relative movement of what?
  11. Can't do Calc: What are my options?

    sorry to infuriate you... but i still feel the same way.
  12. Jan 23-24th Nor'easter Nowcast/Obs

    South Beach, Ocean City NJ
  13. Jan 23-24th Nor'easter Nowcast/Obs

    i am in darien - my power was out already for about 30min i think - i wasn't home it was back on when i got home major road (for darien) is closed around the corner
  14. i race sailboats, so i look at the hrrr wind forecasts quite a lot - i notice that it is frequently initialized with directions completely different than what is observed
  15. Jan 23-24th Nor'easter Nowcast/Obs

    some local wind speeds...
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